Mineralogy and Petrology Notes

Go over syllabus

Play game: Name that rock or mineral1 point for each mineral identified, 1 point for composition1 point for each rock identified, 1 point for story it tells.15-18, including mostly straightforward, but with azurite (striking) and siderite

Physical Properties of Minerals

Crystal faces: faces are planes in the crystal with particular ion/atom densities and arrangements. In general, faces will be the planes with lowest surface energy. However, planes are also affected by growth rates. Are also constrained by the area available for growth.

Faces reflect underlying symmetry of the crystal

Overhead: point out different faces, expressed to different degrees, or growing faster or slower than comparable faces gives more complex appearance, or distorted appearance.

Habit: Malformations, differential growth rates, restrictions of growth areaeuhedralsubhedralanhedral

Form: group of faces that all have the same relationship to underlying symmetry

All images that I didn’t draw are from Klein, 2002 or Blatt and Tracy 1996 unless otherwise indicated.

Can be either open or closed (e.g. pinacoid open, 2 horizontal planes, prismatic is open with any number of planes parallel to one axis, sphenoid has 2 planes intersecting in “hat”)

Form names (not same as form): prismatic, rhombohedral, cubic, octahedral, pinacoidal (2-sided forms)

Twinning: symmetrical intergrowth of two or more crystals of the same substance, often on mirror plane or axis of rotation.

show examples of albite twinning (polysynthetic twinning), show contact twinning (Qz), and penetration twin (carlbad twinning in orthoclase)

State of Aggregation(overhead, point out styles)

Luster, color, streak

luster: way light reflected, metallic and nonmetallic: vitreous (Qz), resinous (sphalerite), pearly (talc), greasy silky (milky qz), adamantine (refractive index)

color: a few are diagnostic (azurite, malachite, turquoise), some vary according to exposure to air (bornite), some by trace composition (Quartz, sapphire, ruby), some by major composition (pyroxene-talk about effect of amount and color)

streak: color of powder, especially useful for oxides. E.g. hematite always has red streak, but color not always red.

translucency: metallic oxides often opaque, most silicates, carbonates, sulfates, others are transparent or translucent if sliced thin enough.

Diffraction among amorphous hydrated silica spheres in opal. (didn’t get to)

Chatoyancy and Asterism

oriented (parallel) elongate inclusions in a mineral can give optical (reflectance) effects. elongate minerals give reflective line perpendicular to orientation of inclusions (chatoyancy). If have three sets of oriented inclusions, can get 6 sided star (asterism) E.g. “star” sapphire, has elongat

Fluorescence and phosphorescence

If you excite an electron to a higher energy state, it can return to ground state in series of small steps in which energy is transferred e.g. to heat (no fluorescence), by releasing a photon (which may be in visible range or not). If the spin state of the electron is different in the ground state than the excited state, the decay is slower, get phosphorescence.

Fluourescence that produces visible light usually results from excitation in the ultraviolet range. The wavelength of light is proportional to the inverse of the energy of the photon. The energy of

the photon depends on the amount of change in the energy level from excited to less excited states.

Cleavage, parting, fracture

Planes of weakness in the crystal, parting is breaking along other planes of weakness, such as twinning surface, exsolution surface.fracture-no planes of relative weakness (Qz)

break along planes with weaker bonds, e.g. Van der Waals bonding in graphite, easily cleaves along that plane.

OrthoPyroxene (Mg2Si2O6): cleavage planes is between more ionic Mg-O bonds, not more covalent Si-O bonds.

HardnessMohs scale

Related to bond strength. Different bonds in different directions, so hardness may depend on direction (kyanite), or which crystal face (calcite)

In general, in increasing bond strength/hardnessVan der Waals, hydrogen bonds, ionic bonds, covalent bonds

Tenacitybrittle, malleable, sectile, ductile

Specific gravitydepends on how closely packed atoms are and atomic mass of atoms. (compare to mass= like dividing by mass of H2O, makes dimensionless, but because water has a density of 1g/cc, you get the same number)

Magnetism (diamagnetic and paramagnetic)

radioactivity, solubility in HCl

Piezoelectric

non-conductor, otherwise it shorts itself out. Must not have center of symmetry (that is, its atomic arrangement is different in one direction from the other along some axis, or polar). Used in a altimeters, pressure gauges, timer in watches and computers. Hydroxyapatite is piezoelectric, important in bone formation.

Mineralogy and Petrology. Physical Properties of Minerals, Lab #1

Mineral hardness:

Mohs Hardness scale consists of 10 “standard” minerals, with hardnesses that increase in a roughly exponential fashion. These are Talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond.

1) Examine the Mohs sample set with the intent of becoming familiar with these (please don’t scratch these samples)

2) Fingernails are roughly 2.2-2.5, a pocketknife or nail is roughly 5.1, a glass plate is roughly 5.5. For each of the following minerals, test its hardness against each of these to see if it is harder or softer. Then, check the actual hardness for that mineral given in your text book to be sure you got it right.

M2 (gypsum)M3 (fluorite)M14 (orthoclase)49-1652 Wards (Kyanite). This sample has a hardness that is strikingly different in different directions. Parallel to the crystal blades it is about 5 (softer than knife or nail), perpendicular to the blades it is about 7 (harder than knife or nail). Test it in both directions until you ‘get’ it. The difference in hardness is due to differences in bond strength in the different directions.

State of Aggregation:

3) Look at the following samples and think about their growth form

Examine the two varieties of gypsum, alabaster and selenite (M1 and M2). Make sure you know which is which. Also look at the satin spar sample 97 Wards in the Mineral Cabinet (from East Bridgeford England).

Use the Figure in your book to identify which type of aggregation or growth character that each of the following might exhibit.

49-1652 Wards (Kyanite), 46-E-4894 Wards (Malachite) This sample is fragile, be careful with it.

Wards (Hematite)Asbestos (25 Wards Mineral Cabinet) This sample is in the locked cabinet.

Twinning:Look at the samples of Staurolite Garnet Schist (R11), Find the twinned staurolite

crystals present in a few samples (called cruciform twins).

Look at the samples of albite we looked at in class. Make sure you can see and understand the polysynthetic twinning. (M9)

Look at the samples of orthoclase we looked at in class. Find a carlsbad twin. (M14)

Crystal faces, cleavage, and fracture:Examine the Quartz crystals (M8). These samples exhibit both crystal faces, and

conchoidal fracture. Quartz doesn’t have cleavage because the covalent bond strength is equal in all directions.

Examine the Pyrite crystals (49E3167 Wards). Also look at the bottom where the fracture (no cleavage) is apparent. DON’T DAMAGE THIS SAMPLE, DON’T SCRATCH IT, BASH IT, WHATEVER. Compare this sample with sample (188 in wards mineral cabinent). Notice the different crystal form.

Look at the samples of Orthoclase (M14). These samples exhibit both crystal faces and cleavage plains. Learn to tell them apart. Two cleavage planes are at nearly right angles. Crystal faces are NOT at right angles. Cleavage will appear in areas of breakage. Crystal faces clearly will not have been broken.

Examine three different samples of fluorite (49E1631 Wards; M3, and 49-1645 Wards). One sample shows crystal growth form (cubic), one shows cleavage (octahedral), and one is a more typical sample. Think about the differences. Look at the atomic structure for fluorite shown in your book.

Examine two different samples of Calcite, M16 and 49E1602 Wards (PLEASE, PLEASE, PLEASE DO NOT SCRATCH, PUT ACID ON, OR CRUSH THIS SAMPLE, OR ANYTHING ELSE BAD!!!!!). M16 shows mostly crystal forms, 49E1602 is a cleavage rhomb. Also look at sample 83 Wards Mineral Cabinet, from Chihuahua Mexico. Notice its hexagonal form. Look at the atomic structure diagram for calcite in your book showing the relationship between the hexagonal and rhombic forms. Think about it. Talk about it with someone.

Examine several samples of m11. Notice the stubby prisms of the pyroxene crystals, as they grew in an igneous rock Also, notice the cleavage. Pyroxene is distinguished from amphibole on the basis of its two cleavage planes at nearly right angles to each other. This shows up most obviously as a stairstep appearance to the cleavage surfaces. Cleavage in pyroxene (Mg2Si2O6) occurs at the ionic bonds between Mg and oxygen, not at the covalent bonds between Si and O. Examine the picture below from your text. lines on this diagram represent covalent bonds between Si and O. Oxygen are represented by the smaller open circles and Si by small black dots. The larger black dots represent octahedrally-coordinated sites in the crystal (called M1 by mineralogists), and the larger open circles represent a structurally-different octahedral site (called M2). Mg goes into both M1 and M2. No lines are drawn for the more ionic bonds between Mg and O. With a pencil, draw how the mineral will break on the right-hand image (don’t cross any

covalent bond lines! Remember that cleavage plains are at roughly right angles when seen on the big scale, cleavage planes pass through the M1 sites, not the M2 sites).

Color and Streak:Look at the samples that are referenced.Color is sometimes diagnostic of a mineral, but usually not. It is diagnostic for Malachite (49E1553, Wards) and Azurite (49H5760 Wards). It is not diagnostic for Quartz (samples M8, and the following samples from the Mineral Cabinet: 45 amethyst, 46 milky quartz, 47 Smoky Qz, 48 rose quartz.

Observe both the color and streak for two varieties of hematite:specular hematite (46E3877 Wards)oolitic hematite (46E3867 Wards)

Density:Two factors influence density: The molecular weight of the ions in the mineral, and the closeness of the packing of ions. Metamorphic minerals often are more closely packed because higher-density minerals are usually more stable at higher pressure.

Examine the samples that are referenced.

M10 (Galena) Dense due to high molecular weight of Pb (PbS). It has the same atomic structure as Halite (M15). Compare them.

46E1022 (Wards) Barite is dense due to the high molecular weight of Ba (BaSO4). Gypsum (M2) is also a sulfate mineral (CaSO4 ۰ 2H2O) but is less dense that Barite. Compare them. Look up the molecular weights for Ba and Ca on a periodic table.

Another sulfate is Anhydrite (CaSO4) Because Ca is so much smaller (as well as less massive) than Ba, the coordination number of Ca (the number of SO4

2- ions around Ca) is much lower than the coordination number for Ba. Therefore, the structures of the two sulfates, Barite and Anhydrite, are very different, even though Ca and Ba have very similar chemical properties (they are in the same column of the periodic table).

Garnet and Kyanite are Alumino-silicate minerals (contain aluminum and silicon bound with oxygen) that have high-densities due to close packing of the ions. They both form under high-pressure metamorphic conditions. Compare them with a lower-density alumino-silicate mineral, Albite.Garnet = M18Kyanite = 49-1652 WardsAlbite=M9

Other Cool Stuff

Birefringence:Due to the electrical fields generated in crystals by the arrangement of ions, light

travels through crystals at different speeds in different directions. When speed changes, the light will bend (refract) just like light bends when it goes from air into water (the “bent pencil” effect when you put it in a glass of water) or like seismic waves refract as they pass through the Earth. This can produce a “double vision” affect.

Calcite (49E1602 Wards): DO NOT DAMAGE THESE SAMPLES!! Place the Calcite rhomb over a sample of text. Rotate the crystal and watch the double images rotate around each other. (Side note: There is one axis in calcite along which this double image effect does not occur: the c axis. This is because light passing in this direction travels the same speed regardless of the direction of light vibration. Trilobites, which had an eye lens made of calcite, had a lens oriented such that it looked parallel to the c axis.)

Exsolution Lamillae: Perthite (46E0514 Wards)Sometimes a mineral that is stable at high temperature, reacts to form two structurally similar but chemically different minerals at lower temperature. This process is called

exsolution (meaning, that the second mineral does not dissolve in the first, but exsolves). Chemically, this is similar to how water will dissolve in air at high temperature, but will condense out at low temperature. We will talk lots more about exsolution later in the course. Feldspar commonly shows an exsolution texture. A composite feldspar at high temperature exsolves to form Perthite, which is a mixture of long, thin laminae of albite (the more Na-rich feldspar) and orthoclase (the more K-rich feldspar). Examine the examples of Perthite until you can spot the whitish stringers of albite and the pinkish stringers of orthoclase.

Labradorescence and Opalescence:

One variety of Plagioclase feldspar, labradorite (if you don’t know what plagioclase is, chemically, and where labradorite falls, then look this up in your book, or in an intro Physical book), often has very tiny exsolution lamilae (too small to see). These laminations form tiny layers in the mineral which will act as a diffraction grating for incoming light. Diffraction is the effect that causes rainbow colors in an oil slick in a wet parking lot. Look at sample 49-1654 Wards (polished Labradorite), as well as the more typical sample 46E4514 Wards. Find the rainbow colors. You should see really striking yellows, greens, and cobalt blues. Sample 46E4514 also has great polysynthetic twinning! Can you find it?

Sample of Opal in the Ore Mineral Cabinet, 213.

Find an opal that opalesces (Not all of them do)The rainbow colors of opal are also from diffraction from layers in the sample. Opal is actually not a mineral because it is amorphous. However, layers are made of tiny round beads of hydrated silicate that form a diffraction grating in a similar fashion to the layers in Labrodorite.

Idea of diffraction. When light “reflects” off of multiple layers, some of the light beams will be in-phase and some out-of-phase when the light emerges from the rock. Whether it is in our out of phase depends on both the angle the light enters and the wavelength of the light. The colors of light that are in-phase will show up as brighter, giving the sample a rainbow appearance.

Elements of Crystal Chemistry

Spectroscopic lines from elements indicates that energy is discrete. Leads to idea of quantized energy states, that is, electrons can’t exist in any energy state, but only in particular ones. The energy of a particular photon is related to the wavelength by the expression E=hc/.

Bohr postulated that electrons exist only in particular shells (or orbits). The more distant from the nucleus, the higher the energy, until the electron escapes from the nucleus entirely.E=-A/n2

where n is the quantum number, related to the mass and charge. Notice that as n goes to infinity, energy goes to 0 (escapes from nucleus). As n goes to 1, E approaches its maximum.

From this, it can be seen that it is easiest to remove the outermost electrons. More and more energy is required to remove inner electrons. Ionization potential: energy to remove easiest-to-remove electron.

hydrogen

Notice, the easiest to remove (such as Li and Na) are those that form positive ions. Those hardest to remove (such as Ne, and Kr) don’t generally form ions at all). Ones like F, Cl tend to form negative ions.

valence electrons, are the outer electrons most easily removed. Produces a charged ION.Elements typically lose a characteristic number of electrons, giving ion a typical valence (e.g.Na+, K+, Cl-, Br-, Ca++, Mg++, Ni++, Sc+++) Some elements may lose a different number of electrons under different conditions, giving it more than one valence (e.g. Fe++, Fe+++, Ti3+, Ti4+,

Go through electromagnetic spectrum and common energy levels absorbed by rotational quantum levels=microwave, vibrational=infrared, electronic (outer electrons=visible, inner electrons = X-ray), nuclear quantum levels = gamma.

Schrodinger model of the atom (briefly)Electron can be thought of as wave like. Schrodinger equation describes position as a wave, predicting only the probability it is at any particular location. The distance of highest probability corresponds to the Bohr distance from nucleus.

4 quantum numbers for electronic energy levels. one corresponds to Bohr’s energy levels (K, L, M, N, O), others orbital shape (s, p, d, f, g), magnetic (determines number of orientations of and shapes, e.g. s=1, p=3, d=5, f=7), and electron spin (only two values, so only two electrons possible per orbital).

Explain how typical ionic charge relates to the number of ions in the outer shell (K, L, M, etc). Examples of Na, Mg, Al, Cl). Explain how it gravitates toward form of noble gases (most stable configuration). Do electron orbital fill exercise. For each atom, indicate its likely valence (charge).

HeLi

Na Ne

Mg Al

O Ar

Types of Bonds

Notice that noble gases are very inert, stable. ionic bond: Transfer of electrons from one atom to another so that both achieve an electronic configuration like noble gas. This is related to filled s and p orbitals in the outer shell. This gives each a charge, and bonding results from electrostatic interaction.

energy = (AZ+Z-/d) Z = charge, d = interatomic distance, A is madelung constant which depends on crystal structure.force (strength of bond) = AZ+Z-/d2)

Which will be stronger, bond between Na and Cl or between Na and I?reflected in melting T: NaCl melts at 801C, NaI at 651 C (melting is when short range order lost).

typical of ionic bonds: Non-conductive (no easy exchange of electrons once noble-gas-like configuration achieved). soluble (many are called ‘salts’), electrons are not shared, but go to one atom, distributed over atom, making bond nondirectional so symmetry of resulting minerals are often high. Moderate hardness (not as strong as covalent bonds, but stronger than other types of bonds). Once dissolved, the free ions provide electrical conductance in the solution.

Often, geochemists approximate energy of crystals from ionic model even when they are not perfectly ionic.

U = N(AZ+Z-/d + sye-d/p)second term is a repulsion term. If try to cram large ion into too small a space, the electrons bump into each other. Since like charges repel, this results in a repulsion term. Repulsion is shorter-range term. There is some “balance” distance (minimum energy).

Side note on my research: Trying to understand and predict how easily trace elements substitute into a particular crystal. I proposed that it could be understood in terms of electrostatic energy:

Relative ease with which different elements substitute into olivine I found that there is a best-fit size about at size of Ni, getting smaller to either side of that, so that both bigger and littler cations fit less well Repulsion energy higher if crammed too tightly, electrostatic energy higher if too large.

Goldschmidts rule: Substitution of one element for another in a lattice: will subst. better if of similar size and charge.

covalent bondShare electrons, such that some electrons do double duty filling outer shells of more than one atom.

e.g. C

Covalent bonds are very strong, very hard (like diamond), high melting T. No free electrons, so do not conduct electricity well.

In reality, all bonds have some ionic and some covalent character.

Some atoms have a strong tendency to attract electrons (electronegativity), others a much weaker ability. The more different two are (one that tends to attract electrons and one that doesn’t), the more ionic the bond. The closer they are, the more covalent the bond.

metallic bondvalence electrons “swim” freely among the nuclei and bound non-valence electrons. The cloud of electrons allows easy movement of atoms (plasticity, tenacity, ductility) and the movement of electrons provides conductivity (both heat and electricity). Weaker bonds yield much softer materials. Only native metals (in nature) exhibit this behavior.

Van der waals bond

These bonds form in neutrally charged atoms or molecules when the motion of electrons becomes synchronized such at one adjacent sides of atoms or molecules gain slightly opposite charges. They are very weak bonds, yielding soft materials and usually low melting temperatures (such as for cooled dimers of Cl2 or O2)

e.g. Graphite

has divalent bonds in plane, with planes bound by Van der Waals forces. The mineral easily cleaves in this plane, making graphite an excellent lubricant. Also used as pencil ‘lead’. Layers for many clay minerals are held together by VanderWaals (e.g. kaolinite, gibbsite, pyrophyllite, brucite, talc).

hydrogen bond

Hydrogen, when it loses its electron, becomes an unshielded proton (positive charge). This exposed positive charge can bond with negative ions, or polar molecules that have a negative pole. Polar molecules are ones that are not the same on all sides, and have positive and negative ends. e.g. H2O. This bond is weaker than covalent or ionic, but stronger than Van der Waals.

briefly mention Pauling’s rules

Mineralogy and Petrology. Thinking about crystal structure, Lab #2

Coordination number:

For ionic compounds, the number of nearest neighbors is determined by the relative sizes of ions. This number is called the coordination number. consider the diagram from your book.

Consider how the coordination of the alkali atom changes in alkali chloride as the size of the cation changes (keeping the size of the anion the same).

Does the coordination number of Cs increase or decrease relative to Na? Is Cs larger or smaller than Na? Is this consistent with the diagram from your book?Draw atoms on the shown faces below for NaCl and CsCl. Use open circles for Cl and filled circles for Na and Cs.

NaCl CsCl

Close Packing:Another way to think of crystal packing is to consider that the anions (usually O2-, but sometimes Cl- etc) are packed in some type of “closest packing” arrangement, and cations then fit into interstitial areas of various shape (tetrahedral, octahedral).

There are two types of closest-packing: Cubic closest packing and hexagonal closest packing. Both of these represent the most closely-packed that equal-sized spheres can be. Consulting with figures 3.37 and 3.38 from your text book (shown in small form below), Use equal-sized marbles to construct three layers of each type of packing (hint, the first two layers is the same for both of them-only the third layer differs).

Polyhedral models:The interstitial spaces between the close-packed anions often have simple geometric shapes (although sometimes distorted), such as tetrahedral and octahedral. Due to its covalent bonding with 4 oxygens, Si often fills tetrahedral spots. 6-coordinated cations (Al and Mg often) occupy octahedral spots. Examine the shapes below to verify that 4-coordination results in shapes with 4 sides (tetrahedral), and 6-coordination results in shapes with 8 sides (octahedral).

These polyhedra can share corners (left two pictures), edges (middle two pictures) or sides (right two pictures). Make sure you understand this and can see it.

Crystal structure can be shown either by ball-and-stick models, showing atoms and bonds, or by polyhedral models, showing the polyhedra formed by groups of atoms. NaCl is shown below in both models, from your book.

Unit Cell:A unit cell is the unit that can be copied over and over to fill up space, thus making up the entire crystal. It reflects the overall symmetry and form of the crystal. On the ball-and-stick picture of NaCl above, draw the boundaries of the unit cell that has the octahedron in it. There is a Na atom at each corner of the unit cell.

How many corners does the unit cell have?

With how many unit cells is each corner Na shared?

How many edges does each unit cell have?

With how many unit cells is each edge Cl shared?

How many sides does each unit cell have?

With how many unit cells is each side Na shared?

There is one atom that is entirely within the unit cell. What is it?

Unit cells for NaCl and CsCl are shown. Notice how they differ. The total number of Na, Cl, and Cs in each are shown. Make sure that you understand how this number is derived by thinking of fractions of atoms shared with more than one unit cell.

CsCl8x⅛+2x½= 2Cs4x½ = 2Cl

NaCl8x⅛+6x½ =4Cl12x¼ +1 = 4Na

ATTENTION: BE VERY CAREFUL WITH THE MODELS. THEY COST OVER $1000!

Halite Model (NaCl)

green = Chlorine (Cl)gray = Sodium (Na)

What is the coordination number (number of nearest neighbors) for Na?for Cl?

Find the octahedra around the Na or Cl. Visualize it.

Find the octahedral planes (there are 4 of them). Rotate such that you see the plane of Na atoms and plane of Cl atoms. These are the octahedral planes.

At what angle are these planes to the sides of the cube?

Relate the chemical composition of Halite to the number of Na and Cl you see in the model.

Halite is in the isometric system. This system has very high symmetry. Examine the block models 1, 2, and 3. Think about how the crystal form is related to the planes of the cube and octahedral.

Beta-Quartz Model

black = Silicon (Si)red = Oxygen (O)

Notice the tetrahedra formed by the four oxygens around each Si. Think about the polyhedral model.

How many sides, corners, edges does each tetrahedron share with adjacent tetrahedral?(the answer, to be read only when you have thought about it, is..............no sides, no edges, 4 corners)

What is the chemical formula for Beta-Quartz, based on the number of oxygens and Si present in the model? (also, think about how many O there must be for each Si, if every O is shared with one other Si). Check its composition in your book to see if you are right.

Framework silicates, like quartz, have tetrahedral that share all corners, and have a ratio of tetrahedral cations (Si), to anions (O) of 1:2.

Notice the unit cell shown on the model. Each unit cell contains 3 Si, and 6O, many of them shared with adjacent unit cells. Make sure that you can count them up, and figure it out! Think about how many unit cells that a particular atom is shared with.

Alpha-Quartz Model

This form of Quartz is a polymorph of Beta Quartz, that is, it has the same composition but a different atomic structure. Alpha-quartz is the form that occurs at lower temperatures (below 500 C). Also, the alpha quartz is slightly preferred at higher pressure, such that at 8kbars pressure it occurs below about 800C. This is because the alpha-quartz is slightly denser than the beta-quartz (the atoms packed together more tightly).

The two forms are very similar. Notice, as with beta-quartz, the shared tetrahedron corners (all four corners are shared).As with beta-quartz, there are no shared sides or edges. The basic shape of the unit cell is similar.

To notice the difference, count the number of Si and O in each unit cell, as you did with beta-quartz, and notice that different fractions of particular atoms are shared with adjacent unit cells.

Rotate the model so that the Si atoms line up. You should be looking at the rhombohedral unit cell from the side. With the model in this orientation, you are looking approximately in the “c” direction of the crystal. When quartz grows into hexagonal prisms, the prisms grow in the c direction, and the hexagonal outline will be perpendicular to the c direction. Do you see any hints of hexagonal form?

Use the 6 two-dimensional rhombohedrons to think about he relationships between rhombs and hexagons.

Make patterns with the rhombs.

Fill up space with the rhombs by translating (moving without rotating) a rhomb-shape. Translate with a 180 degree rotation (the blue dot will not always appear in the same corner).Put two rhombs in a relationship to each other that represents reflection. Imagine that you could draw a line between them such that each side of the line looks like a reflection of the other. There is more than one way to do this.

Make a hexagon with the rhombs. What operations (translation, rotation, reflection), did you have to use?

Make a square and a rectangle with the rhombs (or, can you?)

Forsterite Model (one end of the Olivine solid solution series)

black = silicon (Si)red = oxygen (O)silver = magnesium (Mg)

Try to infer the chemical formula for Forsterite based on the proportions of atoms that you see. Check your book to see if you are right.

Find the Si tetrahedra. What is the coordination number for Si?

Find the Mg octahedra. What is the coordination number for Mg?

Try to visualize the polyhedral model for Forsterite.

How many sides, corners, or edges do Si tetrahedral share with other Si tetrahedral?(And the answer is, not to be read before you think about it.........no edges, no sides, no corners)

Look at the model end-on in such a way that the Oxygen atoms line up. Think about what it would look like if projected onto a flat sheet of paper (the way crystal structures are often shown in books).

Look at the model sideways such that the Mg atoms line up. As above, think about the projection onto a flat page. Notice the “apparent” hexagons around Mg?

Nesosilicates like Forsterite have tetrahedra that do not share any corners, edges, or sides. The ratio of tetrahedral cations (Si) to anions (O) is 1:4.

Illite Model (a mica-like clay mineral, very similar to muscovite and montmorillinite in composition and structure. Use of this mineral name has a problematic history.)

black = silicon (Si)red = oxygen (O)silver = aluminum (Al)aquamarine = hydroxyl group (OH-)gold = potassium (K)orange = other large cations maybe Na, Ca

Find the silica tetrahedra.how many corners, edges, sides do they share with other silica tetrahedral?

Notice that some of the Si has been substituted by Al. Typically, 10-15% of the Si is replaced by Al.

Find the Al octahedrons.How many corners, edges, sides do octahedral share with other octahedral? (the answer is, which you shouldn’t read until looking, is share edges, no sides, no corners)How many corners, edges, sides, do octahedral share with tetrahedral?(share 4 corners)With how many octahedrons is any one octahedron-oxygen shared? (we will talk later in the term about dioctahedral and trioctahedral sheet silicates).

It is very common for sheet silicates (micas and clays) to be made of up various sets of tetrahedral and octahedral layers (covalent or strong ionic bonds) that make characteristic “sandwiches” that are in turn bound by much weaker ionic bonds, or even hydrogen bonds. See if you can find the layers of tetrahedral and octahedral and figure out the pattern.

The stacking sandwiches for illite (which is like muscovite) is roughly the following:

Make sure you can find and see these layers in the model. Think about how the strong cleavage in micas and clays results from this layering. Montmorillinite (the super-water absorbing clays in bentonite) are similar in structure but lack the K layer, having instead much more weakly bonded water layer between the sandwiches.

The c axis is perpendicular to the sheets. Look into the crystal in the c direction (you won’t see the layers). Notice how any plane cutting through the crystal in this direction must cut across the covalent bonds of the tetrahedral and octahedral. Therefore, there is no good cleavage in these directions.

Look how big a unit cell must be!!! Observe how far you go before the crystal repeats, the entire size of the model!

Mineral Reactions, Stability, and Behavior:

Crystallization:

Concept of phases: phases are macroscopically homogeneous regions bounded by distinct edges. gases, liquids, solids are the examples of phases you learn in high school. But a particular material can exist in more than one solid or liquid phase. For example, graphite and diamond are two solid phases with the same composition (polymorphs).

In gases, individual molecules or atoms have no long range order, and are not bonded to nearby molecules or atoms.

In liquids, molecules or atoms have no long range order but are bonded to nearby molecules or atoms but those bonds are not strong enough or persistent enough to maintain a regular long-range order, although a short-range order often exists.

In solids, molecules and atoms are bonded to nearby modecules and atoms, most normally establishing both local and long range order (crystals). Some solid materials do not have long range order (although short range order typically exists). These amorphous materials are called glass.

Crystallization occurs when a material goes from a gaseous or liquid state to a solid, ordered state. This occurs when T, P, composition or other properties change in such a way that the solid state is energetically favored over the former state.

For example, evaporating water from salt water increases the concentration of Na and Cl dissolved in the water to the point that salt crystals will form.

Cooling magma will bring the temperature to a value where crystals begin to form in the melt.

Energy of a crystal is related to the bond energy as well as the arrangement of atoms in the crystal.

We can also think of the bulk energy, the energy of a block of essentially infinite size, and the surface energy, the energy of the material where it encounters something else (air, water, another mineral, etc). Generally, the atoms at the very edge of a crystal are less stable (higher energy).

Think about what the effect of surface energy will have on tiny crystals versus big crystals. (think about volume increases by cube, surface area by square: use example sizes e.g. cube 1x1x1 vs cube 2x2x2 what is surface area and volume of each?)

The surface energy makes tiny beginning crystals less energetically favorable. This is what makes crystals tend to grow into a few big ones instead of many small ones. But it also makes the “starting” step of crystal growth difficult. This step is called nucleation.

E.g. of nucleation in weather. Seeding clouds in the 60’s, still done in some countries. Lowers surface energy. Supercooled air then forms ice crystals or water droplets. Sometimes, air can become supercooled. It is below T at which ice crystals should form, but due to surface energy, they crystals don’t form.

Big perfect crystals, usually form from slow growth, lots of space to grow in to, and ideal growing conditions (such as the T, P, composition are held persistently where the crystals grows slowly at a regular rate). They are rare.

Phase Diagrams, graphical illustration of crystallization reactions and phase transitions.

One-component reactions (different phases of a single chemical component)

Primary variables are T and P.In general, the phase preferred at higher pressure will be the denser phase.The phase preferred at higher temperature will be the less well-ordered phase and/or the phase with higher energy bonding.

Parameters other than T and P can also affect equilibrium, and could be plotted, but are generally not significant in natural situations (e.g. magnetic field, gravitational field, electrostatic field, etc.).

Water overhead: Phase diagrams illustrate fields of T and P where phases are stable. Lines represent reactions, such as the reaction in which liquid water freezes to ice (find that reaction). The triple point is invariant, meaning there only one T and P where all three (liquid, gas, solid) can coexist. Lines are univariant, curves of T and P where e.g. liquid and gas can coexist. Critical point is T and P beyond which liquid and gas are not separated by a distinct phase transition (they become like each other). Based on the ideas discussed above, which is more random, and/or has higher energy bonding, liquid water or vapor water? Does this make sense? Which is more dense, ice or liquid water? Does this make sense? Below 6 millibars, what happens to ice as you heat it up? This is the state of H2O on most of Mars surface.

Metamorphic reactions among different polymorphs of Al2SiO5. Which is the most dense? Which is the least ordered and/or highest energy in bonds? Which is the least dense? Which is the most ordered and/or lowest energy in bonds?Which would form in a contact metamorphic aureole? Which would form in dynamic metamorphic environment (like a subduction zone)?

Broader H2O phase diagram, overhead.Which is denser, Ice I (normal ice that we know), Liquid H2O, Which is denser, Ice I or ice III? How about Ice VI? Which is denser liquid water, or ice VI?Is ice deep inside a moon of Jupiter likely to have a density of less than, equal to, or greater than 1g/cc?

C phase diagram, overheadWhich is more dense, graphite or diamond? Which is less ordered and/or more energy in bonding? Which is more dense, C melt, or diamond? Which is more dense, graphite or C melt? In which will the atoms of C be packed more closely, diamond or Carbon III? Notice the C vapor. What will happen to vapor at a single T if pressure increases?

SiO2 phase diagram overhead.At pressure of around 10kbars (about 29 kilometers depth), what will happen to pure SiO2 as T falls from around 1800 (tell the story). What would happen at about 3 Kbar?Stishovite generally forms in meteorite impacts, is a fingerprint for impact. 80-90 kbar is a pressure 250 km deep or more, where SiO2 generally does not occur as a distinct phase.

CaCO3 overhead.Which is more stable at high P, aragonite or Calcite? Which is the more dense structure? Which is more stable at low pressure? Why does aragonite occur is many gastropod shells?

2-component Phase diagrams (two compositional components)

Can only easily show 2 variables on 2-dimensional page. With only 1 component, you can show both T and P and composition doesn’t change. With 2 components, can’t easily show both T and P as well as composition. So often show a diagram that is valid at only a single pressure (often 1-atm pressure).

Solid-solution series: (e.g. olivine and plagioclase)

Same structure, but Fe substitutes for Mg as go from Forsterite to Fayalite.Above both curves, there is a single phase, melt, that has the composition of the bulk material.As T decreases, the upper curve is encountered. It is called the liquidus, the temperature at which all solid disappears during melting, or where the first solid appears during cooling.At this T, solid olivine begins to form. You can determine the composition of that olivine (remember, it’s a solid solution) by drawing a horizontal line at that temperature. The intersection of the horizontal line with the lower curve (called the solidus) indicates the composition of the olivine.

As T continues to fall, the composition of both the residual melt and the olivine solid solution must change. At any T, the equilibrium composition of melt and solid is indicated by the intersection of the horizontal line with the liquidus and solidus respectively.

Eventually, a temperature is reached where the solid olivine has the same composition as the bulk starting material. At this temperature, the last of the liquid material will solidify (or, if we are melting solid material, it is at this temperature that the first melting will occur).

What reaction does the liquidus and solidus lines represent? (liquid olivine = solid olivine).

Notice that, in general, solids in 2 or more component systems will not have the same composition as the bulk liquid. Therefore, as the solid crystallizes, the composition of the residual melt must change since the total of solids+liquids must always equal the initial bulk composition.

How many phases are present above the liquidus?How many phases below the liquidus but above the solidus?How many phases below the solidus?

Handout of Plagioclase phase diagram, one for each person.

Questions:Which melts at a higher T, pure anorthite or pure albite?What happens to the melting T of Albite as you add more Ca-Al to it?What happens to the melting T of anorthite as you add more Na-Si to it?Consider 40%An, 60% Ab. At what temperature would such a mineral begin to melt? (about 1229C)If it was all completely melted, at what temperature would it start to freeze? (about 1413C)At what temperature would it completely freeze? (about 1229C)What would be the composition of plagioclase at about 1413C when the first plagioclase crystals start to form? (about 76.2% An)What would the composition of the melt be at about 1413C when the first plagioclase crystals start to form? (40% An)Are the plagioclase crystals more Ca rich or more Na rich than the melt? Is that always true? So, if the solid has more Ca in it than the melt, how must the melt change as more plagioclase crystallizes from it?Will the composition of the plagioclase stay the same once it starts to crystallize, or will it change?How will it change? (more Na rich at lower T)what is the composition of the melt at 1300C? (15.3% An)What would be the composition of plagioclase at about 1229C when the last melt solidifies? (40% An)What would be the composition of plagioclase at about 1300C? (54.3% An)

Non-solid-solution binary systems: phase diagram overhead.Pick a couple of compositions and decrease T, showing first phase to appear on liquidus, zone of freezing, and encounter of solidus. Two different phases on the liquidus, depending on the starting bulk composition.Last drop of melt will always be at the invariant point where liquid, phase A and phase B all coexist (remember, other invariant point was where three things coexisted).

Albite-Qz phase diagram overhead.

What form of quartz if went to even lower T? (high quartz then low quartz) (show other phase diagram if necessary)What if at higher Pressure? what would be different? (high Qz instead of cristobalite and tridymite).

Didn’t get to the following, but will probably do these when we cover igneous rocks.Two solid solution series plus a subsolidus exsolution curve. See Albite-Orthoclase overhead, and also draw a simplified schematic version on the blackboard. Note where various phases occur, including polymorphic transitions. High albite, less ordered Si-Al, low albite has more ordered Si-Al.

If slow cooling occurs, microcline occurs in rock. More rapid cooling from higher T results in orthoclase, or even sanidine.

Bunny rabbit overhead with simplified schematic on blackboard. Effect of pressure (H2O pressure) on the curve (5 kbar H2O). Explain how this results in a single feldspar at low water pressure, and two feldspars at high water pressure. Perthite forms when crystallizes at low P, then cools below solidus curve. If the rock cools at depth with H2O, 2 feldspars form to start with and perthite does not occur.

Ternary systems (overhead):plot three components, with temperature plotted as contour lines. Composition is resolved as illustrated.

Other types of phase diagrams. phase diagrams in which two compositional variables are shown (at T and P are constant), are called fence diagrams. Often, pH and Eh are the compositional variables.

Mineralogy and Petrology. Mineral identification, occurrence, and properties, Lab #3

For each of the listed minerals (58), you should create a neat record of the mineral’s features (including composition, crystal class, typical crystal form, hardness, color, streak), and its occurrence (what type of rocks it is found in, under what geological conditions it forms). You should also examine the examples of this mineral that we have in the lab, and make notes about your observations. You can use these notes on exams. This lab is due in 3 weeks. At 3 hours of lab per week, this gives you about nine to ten minutes per mineral. Feel free to spend more time on some and less on others. You have already looked at many of these in Lab #1.

You might use a form like the one on the attached page:

All the minerals listed below should be available in the mineral cabinet (mc), the ore mineral cabinets (omc), or the introductory mineral cabinet (imc). You will have to identify the imc samples in order to find them, which is a good exercise anyway. There are multiple samples for many of the minerals. You should look at all the different samples, because they won’t all look the same. Please don’t scratch, streak, or apply acid to any samples other than those from the introductory mineral cabinet.

Oxides:cuprite (omc 21), corundum (omc 201, 202, mc 54, 55, imc), hematite (omc 46, 47,

48, 49, imc, imc, mc 61, 62, 63, 64), ilmenite (mc 59), chromite (omc60, mc 58), cassiterite (omc 39, 40), magnetite (omc 44, 45, imc, imc, mc 56, 57), pyrrhotite (omc15, mc 70, imc) bauxite (omc 41)

Sulfides:bornite (omc 17, imc), galena (omc 26, 27, imc), sphalerite (omc 27, 32, 33, imc,

imc), covellite (omc18), cinnabar (omc73), stibnite (omc65, imc), arsenopyrite (omc 67, imc-THIS IS A POISON-WASH YOUR HANDS)

Sulfates:barite (omc 82, 176, imc, mc 99), gypsum (mc 95, 96, 97, imc, imc), anhydrite (mc

98)

Carbonates:calcite (omc 36, 179, mc 83, 84, 85, 89, imc), siderite (omc 54, mc 93, imc),

magnesite (omc 112, mc 94), cerussite (omc 29), dolomite (omc 28, mc 91, 92), malachite (omc 23), azurite (omc 22)

halides:halite (omc 182, 183, imc, mc 100)

Silicates:nesosilicates: olivine(forsterite-fayalite) (mc 26), garnet [almandine omc 203, imc,

mc27, mc28; andradite mc29; grossularite mc30], andalucite (mc 35, 36), silliminite (omc 110, mc 38), kyanite (omc 111, mc 37), staurolite (mc 34).

sorosilicates: epidote (mc 31)cyclosilicates: beryl (omc69), tourmaline (omc 181, mc 39, 40, 41)inosilicates: pyroxene [wollastonite mc29, diopside-hedenburgite (mc 18, 19),

augite (mc 20, imc), enstatite-ferrosilite (mc 17)] amphibole (mc 22, 23, 24, 25, imc)

phylosilicates: micas [biotite (omc 172, mc 15, imc), phlogopite (omc 171, mc 16), muscovite (omc 170, mc 13, imc), chlorite (mc 75), lepidolite (omc 86, mc 14, 41)], talc (omc 173, 174, 175, imc, mc 78, 79), kaolinite (omc 99, 100, mc 73), serpentine (mc 76, chrysotile, omc 166, mc 77)

tectosilicates: quartz(omc1,2, 4, 180, 208, 215, 216, mc 44, 45, 46, 47, 48, 49, 50, 51, 52, imc) , feldspars [orthoclase (imc), microcline (omc 107, mc 3, 4), sanidine (mc 1), albite (omc 107, mc 5, imc), anorthite (mc 8)] leucite (mc 12), nepheline (omc 43, mc 10), sodalite (mc 11)

Mineral: Chemical composition:crystal system (and Hermann-Maugin symmetry if you choose):typical form and appearance:hardness:color and streak:

Typical occurrence (include rock types and geological environments of formation)

Notes on your observations of this mineral (include how many different types of samples you looked at).

Mineral: Chemical composition:crystal system (and Hermann-Maugin symmetry if you choose):typical form and appearance:hardness:color and streak:

Typical occurrence (include rock types and geological environments of formation)

Notes on your observations of this mineral (include how many different types of samples you looked at).

Crystal symmetry:Crystallography is (arguably) the second oldest science, after astronomy.Crystals can be thought of as made of motifs (a unit pattern of atoms) which are periodically repeated to construct the entire crystals. The periodic array of points to which the motif is copied is called a lattice. Imaginary lines constructed between these points will enclose only a limited number of shapes. The unit cell is one possible repeating pattern which can fill up space. The environment around each unit cell will be identical to that around all other unit cells. The motif, and the lattice, will have symmetry and dimensions which is characteristic of each crystal.

Symmetry operations (illustrated in 2-D, for 3-D illustrations, see your book)

Rotation:

2-D: 1 fold, 2 fold, 3-fold, 4-fold, 6 fold axes of rotational symmetry

Reflection:

Can consider mirror planes. Some motifs may have more than one. Consider mirror planes in the 2-D figures:

Inversion (in 2-D it is like rotation, but is more complex in 3-D):

Rotoinversion (in 2-D it is like a rotation, but is more complex in 3-D):

The symmetry above can be related to the symmetry of a particular motif. However, to fully understand a crystal, we also have to consider the operations by which the motif is copied through a crystal.

Translation and glide.

Translation and screw (only occurs in 3-D)

Consider the following 2-D motifs. What symmetry do they have? (i4; i4mm)

Hermann-Maugin notation (or the international symbols)

numbers refer to axes of rotation. e.g. 222 refers to three separate axes of rotation, each of which is two fold. Show with an orthorhombic box. A bar over the number designates an axis of rotoinversion (rotation and inversion).

m refers to mirror planes. e.g. 4mm refers to a four fold axis of symmetry with mirror planes in two different orientations (it could be more than two mirror planes). 4/m refers to a mirror plane that is perpendicular to the 4-fold axis. i refers to a center of inversion. It is usually not listed if higher orders of symmetry are present.

Only certain shapes fill up space (lattice systems or crystal systems). Of those shapes that fill up space, only certain organizations of points within the lattice are possible (lattices).In addition, only certain types of motif symmetry are possible (point groups, or crystal classes)Considering the different types of possible lattices and the different point groups, and considering the way that unit cells can be moved by translation, glide, and screw, there are only a limited number of possible types of crystals (plane groups or space groups)

2-D 3-Dlattice system or crystal system

4 6 these are the basic shapes

lattices or Bravais lattices

5 14 number of non-identical periodic arrays of points

point groups or crystal classes

10 32 number of different symmetries possible for motifs.

plane groups or space groups

17 230 combines the number of point arrays with the different symmetries, and the effects of translation, glide, and screw

For example, in two dimensions the 4 basic shapes are square (a=b, =90º), rectangular (a≠b, =90º), oblique (a≠b, ≠90º), and hexagonal (the shape is a rhombus, a=b, =60º). Although other shapes can fill up space, all other possible shapes are equivalent to one of these.

Examples of the different levels of crystal organization for a two dimensional square shape:lattice system square, has only one lattice type (square)

it has two point groups. (4mm, and 4). 4 refers to axis of rotation, first m to vertical and horizontal mirror planes and second m to diagonal mirror planes: The illustration of possible atom configurations is symbolic, showing possible symmetries.

The square lattice has three plane groups (analogous to space groups in 3-D). These include the ways that the square shape can combine with point groups, including also the operations of translation, glide, and skew.

p4, p4gm, and p4mm. 4 refers to the axis of rotation, the first m to the mirror planes perpendicular to the sides of the square, the second m refers to diagonal mirror planes, either through the corners (for p4mm), or between the corner and center point (for p4gm), g refers to glide planes parallel to the sides but between the center and the sides.

For comparison in 3-D, in the cubic (isometric) system, there are 5 different crystal classes (compared to 2 in the 2-D point groups), and 36 space groups (compared to 3 in 2-D squares).

Consider the highest symmetry example: P432. it has 3 4-fold axes of symmetry, 4 3-fold axes of symmetry, and 6 2-fold axes of symmetry (show with a cube). There are lots of mirror planes, however these are not given in the Hermann-Maugin notation because the mirror plane symmetry is already implied by the rotational symmetry.

The six crystal systems in 3-D(from least to most symmetry)

show lack of symmetry with parallelogram in 2-D (although point out rotation axis).

Crystallographic notation for planes, Miller indices - go through cubic and octahedral examples for isometric system only.

Consider the intersection of the plane of a crystal face (or a planar feature within a crystal or unit cell) and a line drawn perpendicularly through that plane from the origin of the a, b, c axes within the figure. Take the inverse of the point of intersection for each axis, a, b, and c. Adjust the points of intersection such as to yield only integers and only integers with no common denominator other than 1. This is the Miller Index for the plane.

Example in Isometric system: planes of the cube and planes of the octahedral expression of that system. e.g. first figure: intersection of b axis is at 1, a and c are at ∞. 1/1 = 1, 1/∞ = 0. Second figure: intersection of a, b, and c are all at 0.5. 1/0.5 = 2. These have 2 as a common denominator. Dividing by 2 yields (111). A bar over the number indicates it is negative.

Example Test Exercise Questions for the section on Symmetry and crystallography.

Illustrate a motif in 2-D that has a three-fold axis of rotation and 3 mirror planes.

Illustrate a motif in 2-D that has a three-fold axis of rotation but no mirror planes.

Illustrate a rectangular figure in 2-D, with motifs at the corners, that has only a single mirror plane.

Illustrate the three 2-D point groups for the square, using motifs different from those we used in class.

Illustrate the similarities and differences between the orthorhombic and tetragonal systems.

Draw a perspective view of a cube (isometric system). Use dashed lines for edges that are hidden. For each of the 2-fold axes of symmetry, put a dot where the axis emerges from the cube. Number the dots such that the two dots associated with each of the axes have the same number (i.e. the first axis has two dots each labeled with a “1”, etc.).

Match the space group (hermann-maugin notation) with the appropriate crystal system. Choose from the following crystal systems for eachisometric, orthorhombic, tetragonal, hexagonal (not rhombohedral), hexagonal (rhombohedral), monoclinic, triclinic. (NOTE: These can be figured out simply from the rules that I discussed in class for each crystal system)

P432 __________________________

P6/m __________________________

Pmm2 __________________________

P2/m __________________________

P422 __________________________

P4mm __________________________

P3m __________________________

P622 __________________________

P23 __________________________

P32 __________________________

P4 __________________________

P1 __________________________

Pm __________________________

P6 __________________________

Match the indicated faces with the proper miller index (three faces in the isometric system). Presume that the “a” axis is emerging from the figure (negative = going into the figure) and the “c” axis goes upward (negative = downward). Positive “b” axis is toward the right. Possible indices are (100), (111), (110), (211), (222), (010), (001), (101), (110), and (422). Mark each of the lettered faces.

Classification of Minerals:

Crystal structure and symmetry are not the only important characteristics of a mineral. Chemical composition is also important. Minerals are often classified into mineral groups.

Mineral groups are based on the primary anion (not cation) of the crystal. This is because minerals with a common cation usually have more in common in terms of properties than do minerals with common cations (for example, compare cerrusite and siderite to galena and pyrite). Also, the anion more consistently reflects the geological environment of formation. That is, sulfides tend to occur together in one type of environment, whereas carbonates occur together in a different environment, and silicates in a third environment.

Native elements (metals and nonmetals) (no anion)e.g. Cu, Au, Fe, Fe-Nie.g. S, Cbonds are metallic in metals, or covalent or other in nonmetalsCrystal structures for the metals are often based on closest packing structures like

hexagonal or cubic closest packing (12 nearest neighbors), or other simple packing structures like body-centered cubic (8 nearest neighbors). Structures in S or C are controlled by covalent bonding angles and typically have 3 or 4 nearest neighbors.

Degree of solid solution is primarily controlled by similarities of atom size, thus Au and Ag have complete solid solution, but the much smaller Cu does not dissolve significantly in Au or Ag. Fe and Ni substitute fairly readily for each other, being of very similar size. etc.

Sulfides (and sulfarsenides, aresenides, antimonides, selenides, and tellurides) (S, As, Sb, Se and Tl are anions)

e.g. FeS2 (pyrite or marcasite), ZnS (sphalerite or wurtzite), arsenopyrite (FeAsS - arsenic substitutes for S), CuS (covellite), Cu2S (Chalcocite), Cu5FeS4 (bornite). Other cations can include the metals cobalt (Co), nickel (Ni), molybdenum (Mo), silver(Ag), cadmium (Cd), tin (Sn), platinum (Pt), gold (Au), mercury (Hg), tellurium (Tl), and lead (Pb) (for example), or the semimetals arsenic (As), antimony (Sb), and bismuth (Bi).

bonds are mainly ionic, although there are also covalent bonds and metallic bonds.

Usually opaque with distinctive streaks and colorsStructures can often, but not always, be thought of as metals in octahedral or

tetrahedral coordination in the interstices between S anions (polygons often are distorted though).

associated with low Eh environments, with high S. Is usually aqueous, often hydrothermal. Is a primary ore-forming mineral group, especially for Cu, Zn, Pb, Ag, Hg, and many others.

Structures of Sphaelerite, Chalcopyrite, and Wurtzite.

Note that Sphaelerite and Wurtzite differ in that the Zn cations have a face-centered cubic arrangement in sphaelerite, but a hexagonal closest packing in Wurtzite (remember that these differ in that the third layer up is different for hexagonal closest packing, but returns to be like layer one in cubic closest packing).

Chalcopyrite differs from sphalerite in that Fe and Cu replace Zn. Because the top and bottom atoms in the sphalerite-sized cell are different (Fe and Cu) and thus adjacent cells wouldn’t have exactly the same environments (criteria for unit cell), the actual unit cell for Chalcopyrite must be twice as big. Notice that you couldn’t stack just half-cells on each other and have it make sense because you would end up with a half-Fe-half-Cu atom between the cells.

Structures of Pyrite and Marcasite

WurtziteSphalerite Chalcopyrite

Pyrite is isometric (2/m3, most common crystal types are cube, pyritohedron, and octahedron), Marcasite is orthorhombic (2/m2/m2/m, crystals often tabular). Pyrite has structure like Halite with covalently-bonded S pairs occupying Cl positions and Fe in Na positions. The S pairs in pyrite decrease symmetry from 432. The three-fold axis of rotation (in this case, rotoinversion) is the characteristic symmetry of the isometric class). The three axes of binary symmetry are the characteristic of the orthorhombic class.

Sulfosalts As, Sb, or Bi (semimetals) substitute not for the anion S, but onto the metal lattice sites.

Oxides (and hydroxides)e.g. Fe2O3 (hematite), Al2O3 (corundum), Ilmenite (FeTiO3), Magnetite (Fe3O4),

cassiterite (SnO2), goethite (FeO(OH)).The structure can be understood as deriving from oxygens that take on some close-packing configuration, with metal cations occupying various tetrahedral or octahedral interstitial spaces. bonds are mostly very strong ionic bonds. These minerals are often very hard. Oxides are usually very stable minerals.Oxides are important ore minerals, including Fe, Cr, Mn, U, Sn, Al (although the stability means that substantial energy investment must be made to separate the metal from the oxygen). Ruby and Sapphire are members of this group.

May be grouped either as simple oxides (one metal plus oxygen), and complex oxides (more than one metal cation).Or they may be grouped according to the cation-oxygen ratio (e.g. Divalent cations yield 1:1, trivalent 2:3, or mix of divalent and trivalent 3:4, tetravalent 1:2). periclase-hematite-corundum structure: Hexagonal closest-packed oxygens, with metal cations occupying octahedra.

Considering all octahedral sites, each oxygen is shared with six octahedra. So, what valence charge is associated with each octahedra? (each oxygen contributes -2, distributed over 6 octahedra, each octahedra has 6 closest neighbor oxygens = 2/6 x 6 = -2). Therefore, each octahedron can be filled with a divalent cation (e.g. periclase = MgO), or 2/3 of the octahedrons can be filled with trivalent cations (e.g. Hematite, corundum). Note: periclase has the same structure as Halite....see if you can count the octahedra around a particular Cl ion.

Brucite-gibbsite structures.

OH- groups in place of oxygens. Different charge, but octahedra connect only in a plane, rather than in 3-D like in the oxides discussed above. (the individual planes are

connected by Van der Waals bonds). Therefore, each OH- at the corners of the octahedra are shared with only 3 octahedrons. This leaves an anion charge associated with each octahedron of -2. Therefore, all can be filled with Mg (Brucite), or two thirds can be filled with Al (Gibbsite). These layers are called dioctahedral and trioctahedral layers respectively.

Conditions: how will CuS and Cu2S differ in environment of formation? How Cu2O and Cu2S differ?

Halides The halides include F, Cl, Br, I, etc. e.g. NaCl (halite), KCl (sylvite), CaF2

(fluorite)Bonding is the most completely ionic of any of the mineral groups because the electronegativities of the constituent elements are the most different. This group has the highest crystal symmetries because ions are spherical and bonds are symmetrical. Symmetry decreases as cations of higher valence than 1 are involved, and the bonds become more covalent.Have the characteristics of ionic solids: e.g. low hardness, poor conductors

Carbonates: (and nitrates)e.g. CaCO3 (calcite, aragonite), FeCO3 (siderite), CaMg(CO3)2 (dolomite),

Cu2CO3(OH)2 (malachite), Cu2(CO3)2(OH)2 (Azurite).

Triangular anionic complexes bound more strongly than the complexes are bound to other ions. Each oxygen has a residual charge of -2/3. Bonding of the CO3 group is not as strong as CO2 bond, so in presence of H+, the carbonate group becomes unstable, breaking down to form CO2 and water.

Bonds in the complex are covalent, bonds between complex and metal cations are ionic.

Calcite structure: Like halite, but with CO3 groups in place of Cl and Ca in place of Na. Symmetry of the triangular CO3 groups produces a rhombohedral rather than isometric crystal. Pseudohexagonal structure of calcite derives from the near-hexagonal close packing of the Ca cations.

Other two groups: Dolomite is also rhombohedral, aragonite is orthorhombic. Larger cations (or Ca at higher T) tend to organize into the aragonite-type structure.

Ca very different in size from Mg and Fe. Therefore, there is little solid solution. Dolomite and Ankarite result when Ca and Mg or Fe don’t mix, but occupy distinct layers in the crystal.

As T increases (at high CO2 pressure so crystals don’t become unstable), the amount of mixing increases, and a solid solution exists at sufficiently high pressure.

Sulfates: e.g. BaSO4 (Barite), CaSO4 (Anhydrite), CaSO4 ۰ 2H2O (Gypsum).

non-polymerizing complexes.

Nitrates, Borates, chromates, tungstates, molybdates, phosphates, arsenates, vanadates.

anionic complexes bound more strongly than the complexes are bound to other ions.

Tidbits: borates can polymerize. Nitrates triangular like carbonates.

Silicates:

Key idea: whether and how the silica tetrahedra are connected to each other by strong covalent bonding, or whether the corners of tetrahedra connect to octahedral or other sites occupied by usually-larger cations by ionic bonds.

Show overhead. Key things to note, the sharing of tetrahedral corners (means that two Si are linked to each other by a single oxygen). The number of oxygens shared between tetrahedra determines the Si-O ratio. For example, with no sharing, then each Si has 4 oxygens, if two Si share one O, then each 2 Si has 7 oxygens, etc.comment on the unit cell shown for phyllosilicates. Notice how each corner of the rhombus is surrounded by a “motif” of silica tetrahedrons arranged in a hexagon. Also comment on what I teach in Physical Geology: the idea of tetrahedrons connected in 0, 1, 2, and 3 dimensions corresponding to island, chain, sheet, and framework silicates.

Nesosilicates: olivineremind of model that we looked at. Show overhead. Point out that tetrahedra share corners with octahedra (M1 and M2), not other tetrahedra.

Inosilicates: Single Chain Silicates: The pyroxenes.

Draw on board:

show cleavage, cutting down through M1 sites at angles such that cleavage is at nearly right angles (remind of lab activity).

The clinopyroxenes (monoclinic) and orthopyroxenes (orthorhombic):Draw illustration:

immiscibility gap: Mg and Fe are near the same size, so they mix in solid solution series, but Ca is much different, resulting in a miscibility gap between the Diopside-Hedenbergite series and the Enstatite-Ferrosilite series. Tie lines show coexisting pyroxenes at a particular temperature (Compositions of coexisting pyroxenes can be used as a geothermometer recording temperature of formation).

Primary compositional components:Diopside (CaMgSi2O6), orthopyroxene ((Fe2+,Mg)2Si2O6)but can substitute Na, Al, Fe3+, Li for M1 and M2 sites, and Al on tetrahedral sitesPutting Ca in M2 site, much larger than Mg or Fe, distorts lattice resulting in the lower symmetry of clinopyroxenes (including diopside, augite, and pigeonite).

(note: putting Ca in M1 site also distorts chains, resulting in the even lower symmetry, triclinic, of the pyroxenoid, wollastonite)

Augite: is mostly in the series CaMgSi2O6-CaFeSi2O6, but with some substitution of Na and Al.

But must charge balance. So, for example, if Na or Li is substituted for Ca (2+), a trivalent cation, such as Fe3+ or Al must substitute for Mg or Fe2+.

Tschermacks substitution: Can substitute Al for Mg on the M1 site, and charge balance by substituting Al for Si4+ on a tetrahedral site.

M2 is bigger (show picture of jadeite): Na in bigger M2, small Al in M1. Point out shared tetrahedral corners and where chains are connected by octahedra.

Phyllosilicates (sheet silicates)

Structure of micas:

Similar to the structure of illite looked at in lab (triangles represent tetrahedra in 2-D, diamonds represent octahedra in 2-D).

Muscovite KAl2(AlSi3O10)(OH)2 - K between covalent octahedra-tetrahedra sandwiches, OH substituting for some O in octahedra, Al in octahedra, other Al substituting for Si in tetrahedra. Dioctahedral because Al is trivalent, not all octahedral sites are occupied (2 of 3). Is the Si-O ratio correct? Have to compensate for the Al on the tetrahedral sites. So better to think of the T-O ratio.

Phlogopite KMg3(AlSi3O10)(OH)2 - what has changed? Mg in octahedral sites, it is divalent, so this is a Trioctahedral structure, all the octahedral sites are filled.

Biotite K (Mg, Fe)3 (AlSi3O10)(OH)2 - what has changed? Is this dioctahedral or trioctahedral?

Petrology

Sedimentary Petrology

Terms related to depositionDetrital = transported fragments and particles

Clastic = fragments and particles that may not have been transported.

Chemical and biochemical = precipitated from water

Major rock typesSandstone (20-25%): clastic rock with particles 0.06 to 2mm.Mudstone (65% of sedimentary): clastic rock with particles less than 0.062mmCarbonate (10-15%): usually chemical or biochemical rock made of carbonate minerals,

particularly calcite, aragonite, dolomite and some siderite and magnesite.Evaporites: Chemical rock, usually formed from evaporation of sea water, or terrestrial

alkaline or salty waters, in arid, restricted basins.

First three make up >95% of sedimentary rocks.

Problems in classification: A carbonate might be made of clastic fragments (either transported or not), such as large

fossil fragments in a limestone, or wave-worked shell fragments in a coquina.Variable amounts of clastic clay can mix with carbonate (Marl).

Age distribution of sedimentary rocksHalf of sedimentary rocks are 130myo or younger (Cretaceous or younger).Exponential decline in exposure as go to progressively older rocks.

Does that mean that sedimentary rocks form more commonly today than in the past?No, is related to a roughly constant probability of destruction by erosion, with a certain fraction of older rocks surviving to later time periods.

Common depositional settings

Most sedimentary rocks are deposited in marine (as opposed to terrestrial) environments. This is because oceans constitute a larger fraction of earth’s surface, because marine environments are more likely to be depositional rather than erosional, and because the sediments are more likely to be covered by later sediments rather than eroded.

Due to fluctuations in sea level, shallow marine water invading continental areas has been more pervasive at various times in the past than they are today. These shallow seas are called epicontinental seas.

Depositional Basins (regions either significantly below base level, or where persistent subsidence provides room for deposition over extended time periods.)

activity: in groups, try to identify key plate-tectonic environments, and the general characteristics of sediments deposited in each.

Oceanic Basins: deposits underlain by oceanic crust (basaltic rather than granitic)a few key considerations: water depth affects light penetration, fossil materials are

often pelagic. Deeper, colder water is more acidic, there is a depth below which carbonates are not stable and an even greater depth below which settling carbonates do not accumulate.

Arc-trench system basins: complex system of basins related plate covergence and subduction. a few key considerations: extensive tectonism and metamorphism makes these

regions complex. Often associated with volcanic input. Basins range from extremely deep to not so deep, and may have either oceanic or continental material base. Sediments include mélanges and turbidites, to more fluvial, deltaic, marine as get closer to the continent.

Continental collision basins: basins that develop where continents convergea few key considerations: include elements of ocean basins prior to convergence,

such as ophiolite, and deposits related to the orogeny such as flysch and molasse deposits.

Basins in displaced terrains (exotic or “suspect” terrain):Key considerations: are tacked onto the edge of a continent by plate movements and

so have structural, stratigraphic, and paleontological discordances with the rest of the continent.

Divergent Grabbens: basins that develop during continental divergenceKey considerations: are often on presently-stable continental margins where past divergence of oceanic-basin-formation occurred. volcanics, intrusives common, interbedded with arkosic red beds. In arid climates, evaporates occur.

Intracratonic basins (regions of subsidence in the interior of stable continents)Key consideration: Deposited in epicontinental seas (non-orogenic, shallow water), with very thick sediments grading laterally into much thinner sediments of similar type. (e.g. Williston Basin, Michigan Basin).

Sandstones and ConglomeratesStudied a lot because particles are big enough to see and study.

classified by particle size and type (composition) of particle

Particle Size (phi scale = -log base 2 of particle size in mm-so 1mm = 0, 1/2mm = 2, 2mm = -1 etc)):Gravel = 2mm to 4096 mm (-1 phi to –12 phi) (granule, pebble, cobble, boulder)Sand = 0.062mm to 2 mm (4 phi to –1 phi) (fine, medium, coarse)Silt = 0.004mm to 0.062mm (8 phi to 4 phi) (fine, medium, coarse)Clay = <0.004mm (greater than 8phi)

Classification by particle type (of sandstones):draw on board

Interpreting Sedimentary Rocks: 1) Present is the key to the past (go look at modern environments and carefully document the sediment details). 2) consider what processes occur in different environments and how these must necessarily affect the sediments. 3) consider the setting of the rock (including the pattern of co-occuring features and the stratigraphic position).

Textural Maturity: (rounding, sorting, size, mineralogy) reflects distance and energy of transport, but also degree of working (kinetic energy) in the environment of deposition. Thus, river sediments tend to be much less mature than beach sediments where waves are constantly working the grains.

Rounding: Factors to consider include resistance to mechanical (or chemical) weathering which preferentially takes off exposed corners, but also cleavage which tends to limit rounding. So, mica will never round, feldspar not too readily, but qz, with no cleavage, will round well, albeit slowly because of chemical and mechanical resistance to weathering.

Very high degree of rounding in quartz grains is associated with wind-blown sand, and, to a lesser, degree, with beaches.

Sorting of grains: Small clay and silt is quickly removed from a sediment at its source, larger particles are sorted according to size by the transporting medium. Sorting reveals something about transport distance, but usually more about environment of deposition. e.g. Beach environments are particularly effective at sorting sand.

Illustration of skew and tails, and bimodal distributionThe sorting and distribution of sediments reveals various processes that have acted on a

sediment.

A skew with a tail toward coarse sediments (negative phi), might reveal that the sediment was winnowed by a medium with insufficient energy to remove larger particles. For example, wind can move smaller particles very efficiently, but not larger particles. Sometimes beach sand may have such a skew if it is built on a base that includes coarser sediment that the waves lack the energy to remove.

A skew with a tail toward fine sediments (positive phi), might occur in stream deposits where the coarser sediments are deposited when the river is higher, then, as it wanes, finer sediments filter into the sand, creating a fine “tail”.

A bimodal distribution often indicates two separate processes, such as fine mud deposited from water in a lagoon, with coarser silt or fine sand blown in by wind. Or, bioturbation may mix two vertically adjacent sediments that were deposited in different environments. Bimodality might also reflect the nature of the provenance material.

Grain type:igneous and metamorphic rocks are about 20% Qz, very little clay minerals.Sedimentary rocks are 45% clay, 40% Qz, 6% feldspar, and 5% disaggregated

rock fragments, 4% the rest.

More chemically and mechanically stable minerals survive, and clay minerals are formed by weathering processes (unstable minerals include olivine, anorthite; more stable include quartz and orthoclase, accessory minerals like magnetite and zircon often stable in sedimentary environments)

So grain type can indicate transport duration. However, grain type can carry other information as well.

For example, Feldspars are mechanically resistant but chemically unstable. Therefore, they can be indicators of climate. They weather much less quickly (more like quartz) in a very cold climate, and much more quickly in a wet, tropical environment.

Garnet, kyanite, muscovite are metamorphic minerals, and can indicate a metamorphic Provenance.

Zoned Plagioclase is typical of igneous rocks (explain relative to phase diagram we studied before, what causes zoning: changes in melt composition, crystal composition, and reequilibration rate controlled by diffusion).

Sanidine is a volcanic rock (high T igneous form of K-spar that is less common in granite)

Ilmenite, chromite, augite, and plagioclase are typical of intrusive igneous rocks.Euhedral hexagonal biotite flakes indicate air deposition from volcanic ash (they would

have lost euhedral shape if weathered or abraded).Glauconite, usually indicates formation in a marine environment from fecal pelletsLithic fragments indicate rapid deposition before significant weathering occurs and can

yield significant information about the provenance, depending on the type of fragment (igneous, metamorphic, sedimentary).

Quartz or other grains may retain information about what type of rock they are from, such as presence of quartz overgrowths from a previous episode of sandstone cementation, 120 degree interfaces typical of metamorphic rocks, undulatory extinctions due to strain in crystals (less typical in volcanic quartz than those subjected to deformation), milky quartz is typical of hydrothermal quartz (due to presence of tiny cavities filled with water).

presence of heavy minerals, or their size distribution, can indicate depositional environment. e.g. wind blown sand often has higher heavy fraction (magnetite) than beach sands.

Example studies (from Colson et al. 2004):effect of transport medium on size of equivalent particles. Think about settling rate (not completely the full problem, but helps to understand concept). Think of a block of wood, a grain of quartz sand, and a grain of gold. What will be their relative fall rates in a vacuum? In air? (gold fastest, wood slowest), in water (wood never settles, bigger difference in settling rates of other particles).

Based on the size of hydraulic equivalents, can sometimes infer depositional medium.

Fig. 7. Measurements of magnetite/sediment average sizes compared to the ratio of sizes expected for transport in either air or water medium. Average particle sizes were determined by measuring sizes of randomly selected particles under a microscope. Sizes for magnetite particles were determined by measuring particles separated magnetically from the sample and identified as magnetite under a microscope and measured.

Hydraulic equivalence is defined as follows: let HN = (DV-BV)/(AF)where HN=hydraulic number, D=density of particle, V = volume of particle, B=buoyancy force of medium per unit volume, A=cross-sectional area of particle, and F=force of the transporting medium exerted per unit area. Then hydraulic equivalence is achieved if HN(magnetite) = HN(sediment). Sediment density is assumed to be approximately that of quartz. Lines on the graph illustrating expected (pred. = predicted) relationships for hydraulic equivalence in either air or water are calculated from this relationship.Sample locations:Foxhills at lmr: upper Fox Hills Sandstone from the Little Missouri River (Frye, 1969, section 9, Bowman Co ND) (LM12).Small x-set: Unit 2 at east end of CHS (708-SB-2).Transition: Unit 3 at West Ledge of CHS (707-WL-3).Wl1: Rooted Sandstone west ledge (708-WL-1).Wl2: Rooted Sandstone west ledge (708-WL-2), wl1 and wl2 are from 2 different cross sets of Unit 4 and are 30 cm apart vertically.Marmarth: lowest Marmarth sandstone from about 4 miles south of the bone bed (SS4).Marmarth at lmr: lowest Marmarth Sandstone from the Little Missouri River (Frye (1969) section 8, Slope County ND) (LM5).

marmarth at lmr

small x-set

wl1

fox hill at lmr

transition

marmarth

wl2

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.18 0.23 0.28

mean grain size (mm)

mea

n m

t siz

e (m

m)

measured pred. air pred. water

From the same study. Unit 3 interpreted as shoreline deposit, water worked but with wind-deposited material also. Unit 4 wind deposited. Unit 6 river deposited or deltaic. Note the fine tail in shown for Unit 6.

use overhead for bar graph.

Table 2. Sorting and Rounding Sorting Rounding

(average)

Unit 3707-WL-3

0.85 2.2

Unit 4708-WL-1 and 2

0.6 2.6

Unit 6712-BB2-9

1.2 1.6

Simple sorting measure (Inman, 1952), 5=rounded, 1=angular.Determined by observation and measurement

of loose grains under a microscope

Sediment analyses from the science center cutbank (student research). Note the good sorting of inferred beach sand, the tail on several water-washed glacial drift, the poor sorting of till, the good sorting and fine grain size of wind-blown sediment. (use overhead)

Mudstones

Due to small size (clay minerals often can’t be studied effectively even under microscope), it is often difficult to extract information. The particles are too small to be meaningfully separated by a sieve.

Grain size. The proportions of clay and silt can be used to infer environments. For example, deltaic sediments are typically silty where some stream flow or water movement occurs, whereas lagoonal sediments may be more clay rich. The proportions of silt and clay can be measured by disaggregating the sample (which may not be easy), and measuring the settling rate of sediments in a column (an hydrometer analysis). problems: 1) you may not completely disaggregate the rock2) The particles may have originally settled as flocculants, or been deposited as fecal pellets, thus the disaggregation may not give you a measure of the actual hydraulic particles meaningful in the depositional environment.

Color: Black is indicative of either free organic carbon, or magnetite or sulfides. Free organic carbon indicates higher organic production than decomposition, usually indicating a reduced environment.

Red and brown indicate oxidizing conditions, typically terrestrial (hematite and goethite) (FeO(OH)).

Green color occurs in the absence of ferric oxides and hydroxides due to illite, chlorite, and biotite in typical shales (indicating more reducing environments where Fe is soluble and doesn’t produce redish colors).

Other textures:laminations, indicate depositional processesSoft sediment deformation features (water escape, or overloading features)Bioturbation and pelletizationpreferred orientations of platy or linear grains (preferential axis of elongation of

paramagnetic minerals can allow orientation to be measured magnetically rather than microscopically)

fissility (depends on orientation of platy minerals and organics, decreases with increasing flocculation which is maximum in about 2000ppm salinity)

dessication cracks.

Example: Pierre Shale study

Bentonite (colloidal silica and smectite, an expandable clay (Van der Waals and hydrogen bonds between layers of silica tetrahedrons-weak bond allows H2O into the layer). Indicates volcanic contribution. Also Crystobalite (determined by X-ray

diffraction), a higher T, low P version (see phase diagram from above) of SiO2, indicative of volcanic origin.

Source direction identified. And further evidence of volcanic input (increase in Feldspar)

Limestones and Dolostones:

Grains (allochems)

fossils: pelecypods, brachiopods, ostracods, crinoid, bryozoans, etc Shape identifies in hand sample, internal structures and characteristic shapes identify under microscope

ooids: sand size concentric, spherical polycrystalline carbonate grains. Form by repeated layering of a nucleus (e.g. qz, fossil), need to be agitated wo typically indicate shallow environment, wave-swept shoal, often cross bedded, other sandstone features.

peloids: spherical or ellipsoid carbonate mud pellet with no internal structure. Often fecal pellets (giving uniform size). Certain boring algae can destroy internal structure of other types of particles, creating a peloid.

Limeclasts: fragment of lithified or partially lithified preexisting limestone. Intraclasts, is a subset of limeclasts that originate penecontemporaneously from within depositional basin, transported e.g. by storm.

Matrix (orthochem)Micrite: clay-silt sized particulate calcium carbonate deposited with grains. Origin

often is algae, or chemical precipitation from sea water. Usually deposited as aragonite that later converts to calcite.

Spar: Crystalline calcium carbonate grown in voids in rock, or recrystallized micrite. Origin is often inorganically precipitated CaCO3 during

Classification

Folk: first name based on allochems, primary name based on matrix (orthochem). E.g. oosparite, pelmicrite. Add additional modifiers: trilobite intrasparite, crinoid biomicrite, etc.

Dunham: first name again based on allochems, primary name based on wether the rock, absent the matrix, is grain-supporting.

Mud supported= mudstone (less than 10% grains= wackestone (more than 10% grains)

Grain-supported = packstone (contains mud)= grainstone (no mud)

original components bound together during deposition (such as scleractinian corals) =boundstone

Dolomite formation has long and controversial history. There is no solid solution between calcite (or aragonite) and dolomite, although Mg solubility is a function of T. Dolomite and limestone do not occur interspersed with each other. Dolomite is associated with limestone and evaporites. Probably more than one mode of formation, postdating deposition.

Mineralogy and Petrology. Sedimentary Rocks and their Interpretation, Lab #4

sample numbers such as 26A refer to samples collected at MSUM pre-Colson and are in rock cabinet 1. samples such as 47E4674E are samples from Wards, some of which have thin sections with them. They are also in cabinet 1. Samples such as W48 are part of the Wards rock sample set and are in rock cabinet 2. IRS refers to introductory rock samples, and are kept either in the metal rock cabinet, or nearby it. I list 10 samples to look at more carefully, but for the samples labeled with “also look at....”, you can just go through the cabinets and look at stuff if you want. DON’T PUT ACID ON THE SAMPLES FROM CABINETS 1 or 2!

Thin sections for this lab are kept separately with the microscopes and have various sample labels. PLEASE TAKE CARE OF THE MICROSCOPES AND THE SLIDES!!!!

Graywacke and Arkose: Both of these rocks are composed of immature sediments, angular, poorly sorted, with unstable minerals and/or rock fragments. They are both deposited near their source after relative short transport and rapid deposition, and are both often associated with orogenic events..

Arkose: Often derived from a granitic source region, contains muscovite, orthoclase, quartz. Commonly flank mountains where alluvial fans form where streams emerging from the mountains lose energy. Typical red color from Fe-oxide cements (common in terrestrial environments)samples: the IRS arkose sample in the introductory lab case is from the Fountain Formation. It was deposited on the flanks of the Ancestral Rockies, a range of mountains that existed in mid Colorado in the paleozoic. This is a classic arkose.

Note the features consistent with the interpreted environment of deposition and source region.

also look at the Triassic Sugarloaf Arkose from Massachusetts (W50).

Graywacke (26A): Often derived from a volcanic source region. They are often associated with flysch deposits or turbidites in association with convergent plate margins. Note the features consistent with this interpreted environment of deposition and source region. Lack the reddish color typical of terrestrial arkose sediments.

Note the features consistent with its interpreted deposition and source.

Also look at the Upper Devonian Graywacke from New York (Acadian Orogeny) W51.

Sandstones:Sandstones form in environments where energy is appropriate for depositon of

sand sized particles. This can include deposition by wind or water, and deposition in terrestrial or marine settings. A few observations are these: beach and wind-blown sands are usually quite mature (mostly quartz, well-rounded, well sorted). Red coloration from hematite cement often occurs in terrestrial deposits. River sands are usually less mature, with more angular grains, are more poorly sorted due to higher silt fraction (“dirty” sand), may contain more unstable grains like orthoclase, micas, etc. Green glauconite usually indicates a marine source since glauconite often forms from fecal pellets.

Examine the following samples:37B: Navajo Sandstone. Terrestrial, wind-deposited sandstone.cite features consistent with this.

46B: Sandstone, Burning Coal Vein ND. Terrestrial, rivercite features consistent with this.

W48: Glauconitic sandstone. Interpretation?

Also look at the sandstones: W45, W46, W47 (argillaceous refers to clay content), and the sandstone concretion (highly cemented region of sandstone) 24B, 27b. Look at the IRS sample from the Wisconsin Dells. It is a supermature beach deposit. Notice features consistent with this interpretation.

Conglomerates represent deposition in a higher-energy environment than sandstones. Also look at the conglomerates: 29A, 26B, 40B (notice the scoria and obsidian rock fragments!- hematite caliche cement suggest desert-soil related formation), W44.

Shales represent deposition in a lower-energy environment than sandstones. Also look at 2B (green river shale is terrestrial intermontane lakes and streams), 31b, IRS claystone

from the Golden Valley Fm (terrestrial river-lagoon), 38B (El Capitan, marine), W49 (siltstone), W52, W53, W54, 41A

Limestones: Limestones are mostly biochemical, and mostly marine. They represent a variety of environments, including wave-swept beaches or shoals (e.g. oolitic or coquina) various marine shelf or reef (e.g. fossiliferous limestone), and other environments.

Look at the following samples, and interpret their environment of formation W59 crinoid sparite or crinoid packstone (compare with IRSR4 crinoid micrite or crinoid wackestone)

34A (coquina or grainstone)

47E4674 (oosparite or oolitic grainstone)

IRS R6? ! Think about this one. I find it very intriguing. Notice the rock fragments (non-carbonate-try acid on them!), the shells, and the micrite mud.

60 (Fremont Limestone): The red color is from terrestrial mud that washed down into caves formed in the Paleozoic Fremont during the Mesozoic period. This type of mottling from insipient cave formation is not atypical of limestones.

Also look at W63 and 47E4664 (chalk), 47E4609, 90, 86, 25B, W62, W64

Other chemical and biochemical sedimentary rocks. Also look at 35a (Dolomite), 47E4694 and W65 (dolomitic limestone), 38A (halite) 15A and W69 (rock gypsum), 28A, W56, and W61 (chert), 21A (flint), 50A (peat), W55 (Bauxite, formed by weathering), W58(diatomaceous earth)

Thin Sections:Using a petrographic microscope entails not only looking at the sample in polarized light (called ‘Plane’ light), but looking at the sample under “crossed polars” or “crossed Nichols”. Polarized light is light which is vibrating in only a single direction (polarizing sunglasses only allow light through that is vibrating in a particular direction). The light source for a petrographic microscope is polarized. This “plane” light goes through the crystalline samples, which can split the plane light into new light components that may not be polarized in a single plane any more. “Crossing the Nichols” involves inserting a

lens between your eye and the sample which only lets light pass when it is vibrating in a direction perpendicular to the original plane light (this means that if you look into the scope with no sample and with the polarizers crossed, you will see only blackness since none of the plane-polarized light is allowed to pass the “analyzer” lens, which is what the second polarizing lens is called. Because the way that the crystal changes the light depends on the exact details of the crystal structure, the light-polarizing and analyzing capabilities of the petrographic microscope give one all kinds of ways to learn about crystals.

Arkose (with hand sample):Thin section 8. Look at the sample using the lowest power lens. First look at the

sample under plane light (not plain light!). The analyzer lens, which is on the right hand side of the optic column between the sample and the eye lens, should be out. The sample should look mostly clear with reddish brown stringers. You can see distinct mineral grains that make up the rock. The reddish brown streamers comprise the iron-oxide cement between mineral grains. Find it.

There are a few small black minerals. These are opaque (light is not transmitted through them. Find them. Common opaque minerals include oxides and sulfides. Based on the environment of formation of this rock, do you think these are likely to be oxides or sulfides? Why?

There are some brownish or greenish mineral grains that are not part of the cement. Find some of them. These minerals are pleochroic. This means that when you rotate the stage in plane light they get lighter and darker (either brown or green). This is a characteristic feature of two micas, either biotite (brown) or chlorite (green). Notice how the biotite and chlorite are mixed in with each other (as the biotite is reacting to form chlorite).

Notice the poor sorting and poor rounding of the grains. What does this tell you about the formation of this rock?

Now, cross the Nichols by inserting the analyzer lens (push in the lens on the right hand side of the optic column). YOU NEVER HAVE TO ‘FORCE’ ANYTHING ON THE MICROSCOPE....TREAT IT VERY GENTLY!!!!

Rotate the stage a little bit and notice that the grains go alternately dark and light. This has to do with the way that the plane light is being changed by the mineral, and then “analyzed” by the crossed polarizing lenses. When it gets dark, we say that it has gone to extinction. This occurs when crystallographic axes in the mineral exactly line up with the polarizing and analyzing lenses. This allows you to see not just the shape of particles, but the actual individual crystals in each particle. If you look around, you may be able to

find some particles that are made up of many small crystals. These are rock fragments (which tells us about the immaturity of the sediment in this rock).

Notice also the new colors of the minerals. The colors you see when the analyzer lens is in are called interference colors. Interference colors are a result of diffraction that occurs due to the way the crystal interacts with the light and the polarizing lenses. Interference colors are characteristic of each mineral. Most of the minerals you see in this sample will be grey or light yellow. These are quartz and feldspars. You can tell the feldspar from the quartz because the feldspar will be twinned. Find the black and white straight patterns that mark the synthetic twinning typical of feldspars. There is also some carlsbad twins around, which you might spot as being a twin that is on a bigger scale than the small “railroad track” synthetic twins.

Now, with the Nichols crossed, look around for some really bright colored minerals (bright orange or blue or red). Uncross the polars briefly to see if the mineral looks clear in plane light. This is muscovite, another mineral typical of Arkose.

Sandstone (with hand sample)Thin section 7: Begin looking at this sample in Plane light. Notice the much

better rounded grains than in the arkose sample. The reddish brown goo between the grains is hematite cement. (What do these two observations tell you about the formation of this rock?)

Cross the polars and look for feldspar and muscovite (notice that there is not much, if any- what does this tell you about the formation of this rock?)

Cross and uncross the polars a few times, also rotating the stage, and notice that the shape of the rounded grains does not match the shape and size of the actual crystals (each separate crystal being defined by the extinctions of the individual crystals). The “extra crystal” beyond the edges of the apparent grains are quartz overgrowths that formed on the original quartz sand grains. Thus, the quartz overgrowths have filled in most of the original void space in the rock, forming a very strong quartz cement.

There are some void spaces remaining. When you cross the polars, the void space is always black, no matter how you rotate the stage. Find some of these void spaces (now filled with epoxy of course).

There is a second type of “always black” region that is not a void space. If a quartz crystal happens to be oriented so that we look exactly down the “c” axis, it does not change the polarized light, and the result is that the polarized light maintains its same orientation through the quartz crystal and then is “cut off” by the analyzer lens which only lets light through that is perpendicular to the original polarized light direction. Find a quartz crystal that you are looking down the “c” axis at.

Center the “c axis” quartz crystal on the stage, then rotate in the high power lens and refocus (BE VERY CAREFUL, IT IS EASY TO RUIN THE MICROSCOPE OR THE SLIDE BY RAMMING THE HIGH POWER LENSE INTO THE SAMPLE AS YOU

FOCUS!!!). Now, engage a new lens, called the Bertrand lens. This lens is on the left hand side of the optic column. You should see a small cross when you look into the scope. This is called an optic axis figure and can allow one to make important inference about the crystal structure of a mineral. Find a different quartz crystal, one that gets very bright as you rotate the stage, and do the same thing. The cross is still there but it is off center. As you rotate the stage you can watch the different legs of the cross as they cross the field of view.

Limestone (sample D-26) (biowackestone, or biopelmicrite)

Things to find:

Orthochems:micrite and spar (the spar will have bigger crystals of calcite, which will look clear in plane light with high order interference colors (bright reds, blues) with crossed polars). Micrite looks “muddy” and you can’t see individual crystals even under the microscope.

Allochems:This sample has a variety of fossils as well as peloids. Peloids are more abundant in the top half, fossils in the bottom half.

Trilobites: characteristics include a “shepherd hook” shape, and an extinction that sweeps from the center outwards toward each end. Due to recrystallization, the wave-like extinction is only sometimes seen in this sample.

Brachiopods: Characteristics include two layers to the shell (not visible much in this sample due to significant recrystallization of this early paleozoic sample), and a fabric that cuts at a shallow angle relative to the trend of the shell.

Bryozoa: Characteristics include a cellular pattern formed by the openings where each zooid lived (this is distinct from the cellular pattern of coral.....I didn’t find any coral in this sample).

Mollusc fragments: (characteristics include the shell being replaced by calcite crystals, producing a mosaic appearance to the crystals (visible under crossed Nichols as you rotate the stage).

Peloids: Round structureless blobs.

Other things to notice: Several brownish stringers run through the sample. These are mini-stylolites, dissolution features that form when minerals dissolve when the rock is under pressure.

Some conversion to dolomite has occurred. This can best be seen in the red-stained area. The calcite stains red, but the dolomite does not. Notice the rhomb-shape of the dolomite crystals.

Limestone (sample 44-7362) (with hand sample)

Notice the concentric nature of the oolites. These form where waves continually lift and redeposit them, such as a shoal.

You may find pieces of brachipod, echinoderm, or foraminifera at the cores of some of the oolites and coated fragments.

Mineralogy and Petrology. Review sheet for Exam 1.Covering Mineralogy, and Sedimentary Petrology

Sure Bets (80% of exam):(This category basically includes the key concepts that we have learned so far)

Be able to fill electron shells for a selected atom, predict that atom’s typical valence, and explain qualitatively what electron “shells” are.

Be able to describe and explain the different types of bonding, including each of their properties and a description of how the bond works, with illustrations, giving an example of each.

Be able to interpret a single-component phase diagram (P-T), including understanding of the relationship between lines on the diagram and the relative densities and enthalpy-entropy characteristics of phases (that is, we simplified concepts of free energy to think about the higher-T phase being the one that is less ordered and/or has more energy in its bonds).

Based on single-component phase diagrams, be able to predict under what conditions different phases will exist.

Be able to interpret a binary phase diagram with complete solid solution (like Fo-Fa, or An-Ab), including reading off the diagram the temperature of initial crystal formation and temperature of final solidification (for any particular composition), and the composition of melt and composition of solid (for any particular temperature). Be able to predict how composition of the melt will change with either crystallization or melting.

Be able to reproduce a drawing and explanation of each of the six crystal systems, including the key symmetry elements of that group and the angular and dimensional qualities that set them apart.

Be able to read Hermann-Maugin notation sufficiently to identify which crystal system it belongs to, or, given a crystal system, to give an example of Hermann-Maugin symmetry in that system.

Be able to identify the Miller indices for faces in the isometric system.

Be able to predict and/or discuss how ionic size and charge affect substitution in a crystal structure (Goldschmidt’s rules), including the importance of charge-balancing substitutions (for example, I may ask you to figure out if a particular substitution is likely).

Be able to classify and name a sedimentary rock (sandstones, conglomerates, and carbonates primarily) by particle size, or particle type, given the rock’s description.

Be able to interpret what various particle size distributions and/or mineral types in a sandstone might tell you about a rock’s history.

Be able to explain the reflux model for formation of Dolomite. (I am not actually going to include this one on the exam, but I list it so as to be complete).

I will have about 3-7 minerals for you to identify and/or identify basic properties of.

Other Stuff (20% of exam):(This category basically includes the important information and language that we have learned so far......there is a lot, of course!)

I can’t reproduce here all the things that fall into this category. But I have listed some obvious ones to give you an idea of what I mean by “information and language”.

Be able to illustrate, explain, and/or identify twinning, various types of aggregation, crystal faces (and crystal form), cleavage, fracture, hardness, density, streak, habit, birefringence, exsolution, labradorescence, fluorescence.

Especially, be able to explain the difference between a crystal face and a cleavage face.Know what the expression E=-A/n2 tells us about electronic energy levels.Understand the relationship between quantum chemistry and spectroscopic lines.Given their position on the periodic table, be able to predict the relative ionic or covalent

character of particular bonds.Know, conceptually, the difference between the Bohr model and the Schrodinger model

for atoms.Know what electronegativity meansUnderstand what the expression AZ+Z-/d tells us about the energy of an ionic bond.Be able to recognize features in ball-and-stick models, including number of nearest

neighbors (coordination number), presence of tetrahedra and octahedra, shared corners, edges, or sides of tetrahedra or octahedra.

Be able to count the number of other unit cells that a particular atom in a unit cell will be shared with (such as a corner atom, and edge atom, or a side atom in a cubic unit cell).

Know what a phase is.Know what a phase transition is.Be able to draw 2-D plane groups that have indicated symmetries (square system).Know the differences and relationships between a crystal system, Bravais lattice, crystal

classes, and space groups.Draw examples in 2-D of various types of symmetry. For example, be able to illustrate

mirror planes, rotation, and center of inversion. Be able to illustrate an example of a lattice system, a point group, and a plane group in 2-

D (preferably in the square system)Be able to identify key characteristics of Silicates, sulfates, carbonates, halides, oxides,

sulfides, and native elements.Be able to draw the cleavage planes in a pyroxene, given the crystal structure diagram.

Be able to explain the difference between dioctahedral and trioctahedral packing.Identify which type of silicate a mineral is by it ratio of tetrahedral cations to oxygen.

Understand how Al sometimes substitutes for Si and be able to identify this from the chemical formula.

Draw a picture showing the layers of a sheet silicate such as illite or muscovite, including the tetrahedral and octahedral layers, and indicating the cleavage layer.

How do clastic rocks differ from detrital rocks?What are the proportions that different sedimentary rock types comprise of the crust?Why does the proportion of exposed sedimentary rock decrease exponentially with

increasing age?What kinds of deposits might be typical of oceanic basins? arc-trench systems, etc.What does euhedral biotite in a sedimentary rock tell you? zoned plag.? Sanidine? etc.What is bentonite?What does color reveal about mudstone deposition?What is flocculation? How does it influence deposition?What are peloids? ooids, fossils, limeclasts, intraclasts, etc.How does the Folk classification differ from the Dunham classification?What is spar? micrite?

Mineralogy and Petrology. Igneous Rocks, Textures and Compositions, Lab #5

Examine the following hand samples. For each sample, use the rock name, along with information in your book and examination under a hand lens to try to identify major minerals in it (1-4 as indicated). Secondly, list the appropriate key minerals [those from the classification diagram: quartz, plagioclase, alkali feldspar, feldspathoid] with their approximate proportions. Finally, for each sample, identify its key texture and give the most likely interpretation of how that texture occurred. Key textures include such things as aphanitic, phaneritic, porphyritic, vitrophyric, ophitic, poikilitic, vesicular, amygdaloidal, veining, etc.

Calc-Alkaline familyWards 2, muscovite, biotite granite

Identify minerals (4):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 12, rhyolite porphyry

Identify minerals (2):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 27 Hornblende Andesite

Identify minerals (1):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 7, Granodiorite

Identify minerals (4):(be sure to notice the difference in proportion of mafic minerals when compared to granite)Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Mafic/ultramafic family

Wards 25, Diorite

Identify minerals (3):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 30, Olivine gabbro (this sample is missing, if you can’t find it, use the sample of gabbro in the metal cabinet with the introductory physical geology samples.)

Identify minerals (2):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 32, Anorthosite

Identify minerals (1):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 35, amygdaloidal basalt

Identify minerals (1):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 43 serpentinite

Identify minerals (0):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Alkaline family

Wards 13, hornblende syenite

Identify minerals (3):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 18, Nepheline, sodalite, syenite

Identify minerals (4) Note: sodalite fluoresces under black light:

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 42, kimberlite, a mafic alkaline rock, (be sure to notice the xenoliths!!!)

Identify minerals (0):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Wards 23 latite porphyry

Identify minerals (3):

Key minerals from classification diagram, with approximate proportions:

Identify and interpret texture:

Igneous Rocks in Thin Section

Thin Section 3, GabbroFind the Olivine: it has an erratic fracture pattern (not the regular linear fractures indicative of good cleavage). It is a faint greenish color in plane light. Under crossed Nichols it has high interference colors, red/blue/green.

Find Plagioclase: it is lath shaped, or rectangular. It has low interference colors under crossed Nichols (grey/white). It exhibits very distinctive polysynthetic twinning, visible under crossed Nichols by the alternating bands of extinction as the sample is rotated.

Find Biotite: It is brown or green and pleochroic (as in the sedimentary samples). It has a birds-eye texture in plain light (resembles birds-eye maple).

Notice the opaque minerals.

Find the pyroxene: it is a light brown color in plane light. It has medium interference colors under crossed Nichols (orangish). fractures run parallel to each other, indicating cleavage. In many grains, two sets of fractures intersect at nearly right angles (2 cleavage planes at right angles to each other).

There is some of another type of feldspar, generally without polysynthetic twinning, which contains poikilitically enclosed olivine crystals.

Notice the serpentinized veins (small alteration veins): they are greenish in plane light. where the veins intersect opaque minerals or olivine, biotite often occurs. Opaques and olivine are also altered to biotite along the veins.

Thin Section 1, Granite

Find the quartz: equant crystals with low interference colors (grey to yellow). No cleavage planes, usually don’t show an extensive fracture pattern either.

Find the feldspar: there are two kinds. Na-rich plagioclase shows polysynthetic twinning. K-spar grains are quite large and have a mottled extinction appearance under crossed Nichols.

Thin Section 2, Diorite

find the amphibole (hornblende): light brown to dark brown or green, pleochroic, cleavage fractures run at angles of about 124 degrees. Medium (orangish) interference colors.

There is a particularly large hornblende grain. Find it, and notice that it is ZONED (meaning that its compositions changes outward from the middle of the crystal). This is due to changes in the melt during crystallization. Notice also that there are

abundant poikilitic inclusions in the greener-colored rim. The rim is probably richer in Na and Fe than the interior.

There is some biotite. You can distinguish it from hornblende because it will generally have different pleochroic colors slightly, only one dominant cleavage apparent (and pleochroic colors will be darkest when the cleavage plane of the sample is aligned with the polarizing lens), birds eye appearance, and higher interference colors than hornblende (red/blue). Also, the grain will go to extinction when the cleavage planes are parallel to the polarizing lens (parallel to the cross hairs), whereas amphibole goes extinct when there is a distinct angle between the cleavage planes and the polarized light orientation (parallel to the cross hairs).

find the polysynthetically-twinned Plagioclase

Notice than quite a few of the opaques (ilmenite and magnetite) are diamond shaped. Based on their crystal systems, are the diamond shaped opaques more likely to be ilmenite or magnetite?

in Plane light, find some the hexagonal poikilitic crystals in hornblende. These are apatite (check out the crystal system for apatite in your book). Notice that many of the most perfect hexagons are always black under crossed Nichols. This occurs when you look right down the c axis of the crystal.

I also found a grain of titanite (diamond shape, high interference colors), light brownish color.

Wards 44E6119 Diorite Porphyry (compare this to Thin Section 2)

Find the euhedral to subhedral phenocrysts of Plagioclase and Amphibole. There is some biotite. These phenocrysts are in a fine-grained matrix.

Wards 44E7310 rhyolite tuff

Find the subhedral to anhedral SiO2 grains (low interference colors, white/yellow). I think that at least some of these are cristobalite (the high T form of SiO2). This is based on the presence of a “tile” structure seen in some of the grains with crossed Nichols (near the bottom middle when the number is placed to the right). This tile structure is characteristic of cristobalite.

Find the K-spar (some with Carlsbad twinning). (lath shaped often, grey/white in crossed Nichols)

The matrix shows compression/flow fabric (trachytic texture) due to alignment of grains.

There are several vugs, or cavities (these areas can be identified under crossed Nichols because where there are no minerals, the area will be persistently black). These cavities are filled with vapor-growth crystals that are usually fibrous and radiating. These crystals likely grew as vapors from the still-hot tuff filtered up through the newly deposited rock.

Wards 44E 7327 Hornblende Andesite

note the porphyritic texture.

phenocrysts include muscovite (clear in plane light, high interference colors under crossed Nichols and may show birdseye like biotite), plagioclase (lath shape, polysynthetic twinning), hornblende (equant crystals with orange/yellow interference colors)

Many of the hornblende grains are altered (chemical reaction in a late stage of eruption or weathering), and many have reaction rims (corona texture).

round Thin Section 8: Garnet Pseudoleucite Syenite from Magnet Cove Arkansas

This sample is part of a research project that I worked on as a graduate student. You can use the micrometer slide to support the thin section under the scope.

Find the garnet: large brown isotropic crystals (isotropic minerals always appear black under crossed Nichols). These crystals are cyclically zoned (meaning that they have bands of different composition that occur concentrically in the crystal). I interpreted this zoning to be due to periodic drops in pressure in the magma chamber when volcanic eruptions from the chamber tapped its supply of volatiles. Some of the garnet crystals have pyroxene poikilitically enclosed, some with the ends of the pyroxenes “ripped off” These are glomerocrysts, clusters of phenocrysts caught up in the magma during eruption.

Find the hedenbergite (Fe-rich pyroxene): light brown color, orange interference colors.

Find aegerine (Fe-Na rich pyroxene). The aegerine is a later-stage pyroxene, which I interpreted began to form after the magma was erupted to the surface (carrying with it the large garnet and hedenbergite phenocrysts that had grown in the magma chamber). Notice that the hedenbergite sometimes have aegerine rims, consistent with this interpretation (corona texture).

I interpreted that the change in type of pyroxene was due to the change in pressure during the eruption and the loss of CO2 from the magma. Notice that both hedenbergite and garnet have reaction textures of various sorts, indicating that they were no longer stable in the melt once the eruption began.

Nepheline and Alkali Feldspar: The grey stuff. Nepheline is a low SiO2 mineral. These grains are strongly altered, indicating that they also were not in equilibrium once the

eruption occurred. Lath shape and simple twinning usually indicates a feldspar grain, whereas the Nepheline grains tend to be more equant or hexagonal in shape.

Pseudoleucite: Leucite is very easily altered and rarely found. In this rock, you can find shapes in the thin section where leucite existed as a phenocryst in the matrix. These shapes are roughly octahedral in shape, indicating that leucite was there. But the mineral present is no longer leucite (its kind of a grunge of tiny crystals). Thus, it is called pseudoleucite because it has the shape of leucite but has been altered to a different mineral.

Minerals visible in hand sample:25: hornblende, plag, sphene, biotite30: plagioclase, olivine27: hornblende32: anorthositic plagioclase35: Quartz, calcite, epidote/chlorite in amygdules43: magnesite, pyrite, serpentine13: alkali feldspar, hornblende, biotite18: alkali feldspar (with twinning), nepheline, sodalite, amphibole42: serpentinized olivine (porphyritic), large xenoliths.23: plagioclase, biotite, pyroxene

Igneous Rock Petrology

Extrusive Rocks (smaller crystal size, aphanitic)lava flows (basalt, rhyolite)pyroclastics (tuff, welded tuff, volcanic breccia)

Intrusive Rocks (larger crystal size, phaneritic)discordant concordantdikes and sills: more rapid cooling usually than larger intrusives (thought puzzle

on crystal size)plutons, vs countryrock

Cumulate Rocks, subcumulate (crystallized melt between cumulate grains)

Layered Intrusives. Body of magma that crystallizes over time, with different mineral crystallizes at different times, and the composition of the residual magma changing with time. Due to localized crystal growth and/or crystal settling, layers of various minerals develop. (show olivine and chromite layers)

Rock families

Calc-alkaline (typical of subduction and convergent zones, includes granites, andesites, basalts) Typical minerals: pyroxene, amphibole, mica, quartz, plagioclase and K-spar

Mafic and Ultramafic (typical of divergent zones, mid ocean ridges, includes basalts, gabbros) Also related to massive flow basalts, such as Deccan Traps or the Columbia basalt. probably associated with huge mantle plume (like Yellowstone?)minerals: Ca-plag, pyroxene, olivine

Alkaline High alkalis, low SiO2, (Typical of areas where magmas originate at greater depths, such as hot spot volcanism in oceanic areas yielding alkali basalts, or continental divergent areas yielding alkali basalts or syenites to highly alkaline rocks such as nepheline syenites, kimberlites, carbonatites)minerals: Na, K feldspars, feldspathoids (nepheline and sodalite, leucite), biotite

Textures:

Euhedral, subhedral, anhedral

Granitic (grains roughly of similar size but “grown together” such that some are euhedral, some subhedral, some anhedral)

porphyritic (phenocrysts and groundmass)

Graphic texures (intergrown minerals, usually Qz and Alkali feldspar, where qz lies along crystallographic planes in feldspar. Probably a near eutectic crystallization and occurs most often in granites and pegmatites)

myrmekitic (wormlike intergrowths of qz and sodic plagioclase, probably subsolidus, with eutectic composition)

exsolution: At higher T, there is a solid solution, but as T decreases the range of that solid solution decreases. This causes two phases to exsolve as T decreases. Often the exolution occurs along crystallographic planes. Perthite is a common example, Na and K feldspars form a solid solution at higher T, but not at lower T. So single crystal of Na-K feldspar exsolves into thin lamellae of Na feldspar and K feldspar.

Draw a phase diagram showing only the solvus curve for Na-K feldspar)

Ophitic: subhedral augite grains enclose plagioclase crystals, indicating concurrent growth. The tendency of the px to enclose plag is related to growth properties. Often occurs at intermediate cooling rates (fine to medium grain size), such as occur in dikes or sills (slower than basalt, faster than gabbro). Sometimes the word diabase is used to refer to the opposite texture in which plagioclase encompasses augite. Diabase is the name of the basalt-composition rock with ophitic or diabasic texture.

poikilitic: later crystallizing larger crystals enclose earlier smaller crystals.

Trachytic texture: orientiation of plagioclase laths indicating flow orientation or compaction.

Coronas: a reaction rim around a crystal indicating that the melt began to react with the mineral but cooling proceeded too quickly for the reaction to go to completion. E.g early olivine may be rimmed by later orthopyroxene. As the melt crystallizes olivine, it becomes richer in SiO2. Eventually, Olivine is no longer stable and pyroxene grows instead.

vesicles: air pockets formed where gases exsolved from the magma.

Amygdules: ground-water deposited vesicle fillings, zeolites, quartz, calcite, epidote.

macro textures of extrusive lavaspillow basalts (eruption under water), pahoehoe (ropy, billowy surface, flows

rather than moves as a cored mass of fragmented blocks), aa (clinkers on surface, dense interior of flow), block lava (shear results in blocks, less rough than aa)

Igneous Rock Classification:

Norite= Plag + Px, Troctolite = Plag + Ol, ultramafic = more Ol and Px, Anorthosite = plag, usually cumulate, Peridotite= a field term for Ol+Px.

Quartz-Alkali Feldspar-Plagioclase Feldspar-Feldspathoids (SiO2 deficient)

Phase relations and the justification for the divide based on the alkali/SiO2 contents: More complex when consider other elements (K), CO2 and water, and especially pressure.(from Basalts and Phase Diagrams by Morse)

Effects of Textures:PegmatiteobsidianTuff: volcanic sedimentary of ash, glass fragments, variable fusion (welding)Breccia (volcanic sedimentary, with angular fragmentsAplite sugary appearance, lack of mafic mineralsporphyry (50% phenocrysts)

Special rocksCarbonatitespilites: submarine basalt: Book states “All plagioclase has typically been converted to albite and is usually accompanied by secondary chlorite, calcite, epidote, chalcedony, or prehnite. Spilites are thought to have been subjected to submarine hydrothermal seawater alteration...Serpentinite (altered ultramafic)Laprophyres (K-rich, mafic, CO2 rich?)....

Explain the conversion to albite.

Fractionation of Ol and Px and the tholeitiic and calc-alkaline trends: pg 77-78.

Igneous Rocks and Phase Equilibria

Talk about zoning in crystals. Which way will zoning occur (that is, will outside or inside of the crystal be more Ca-rich?). Do same thing with Fo-Fa - which way zoned?Talk about phase rule: F=c-p+intensive variables. Where only T and P vary, this reduces to F=c-p+2. c=components, p=phases.

Non-solid-solution binary systems: phase diagram overhead.Pick a couple of compositions and decrease T, showing first phase to appear on liquidus, zone of freezing, and encounter of solidus. Two different phases on the liquidus, depending on the starting bulk composition.Last drop of melt will always be at the invariant point where liquid, phase A and phase B all coexist (remember, other invariant point was where three things coexisted). Identify eutectic.

Albite-Qz phase diagram overhead.

What form of quartz if went to even lower T? (high quartz then low quartz) (show other phase diagram if necessary)What if at higher Pressure? what would be different? (high Qz instead of cristobalite and tridymite).

Talk about direction of composition change, whether more Si rich or Si poor. Fractional crystallization, mention thermal divide

Talk about Ne solid solution, SiO2 dissolved in Nepheline. No Ne dissolved in SiO2.

See Albite-Orthoclase overhead, and also draw a simplified schematic version on the blackboard. Two solid solution series plus a subsolidus exsolution curve. Note where various phases occur, including polymorphic transitions. High albite, less ordered Si-Al, low albite has more ordered Si-Al.

If slow cooling occurs, microcline occurs in rock. More rapid cooling from higher T results in orthoclase, or even sanidine.

Bunny rabbit overhead with simplified schematic on blackboard. Effect of pressure (H2O pressure) on the curve (5 kbar H2O). Explain how this results in a single feldspar

at low water pressure, and two feldspars at high water pressure. Perthite forms when crystallizes at low P, then cools below solidus curve. If the rock cools at depth with H2O, 2 feldspars form to start with and perthite does not occur.

Reaction between crystal and melt: pertitectic point, The Fo-Quartz diagram. Also talk about liquid solvus, and the thermal maximum as go toward MgO, cristobalite becomes tridymite and quartz at lower T. Figure from Morse, Basalts and Phase Diagrams.

Three components, get ternary phase diagram, each side of the ternary being a binary phase diagram. With 4 components it is normally handled by constructing slices through a tetrahedron (each face of tetrahedron being a ternary phase diagram.

The diagram below is from an assignment that I did for intro to thermodynamics for geologists.

Also, if consider other variables, such as P, can consider shifts at a variety of pressures. (diagram from intro to thermo for geologists).

Magma evolution

Equilibrium and fractional crystallization

Mass balance and chemical equilibrium considered together. Are neither creating nor destroying elements. But chemical processes divide elements into crystal or melt in unequal ways, causing the composition of the crystal and melt to deviate from the bulk composition of the system.

If equilibrium, then composition of both melt and crystal are defined by the phase diagram. If crystals are somehow removed from re-equilibrating with the bulk system, then each melt composition is like a new bulk composition, and can get more extreme changes in melt composition.

Equilibrium “batch” crystallization:Two equations, one for equilibrium, one for mass balance:

Cs/Cl = D (equilibrium, Cs = concentration of element in solid, Cl = concentration in liquid, D is the bulk partition coefficient)

(1-F)·Cs + F·Cl = C0 (mass balance, the number of atoms of element don’t change, f = fraction of original melt remaining, C0 = concentration in the original melt.)

Solving for the two equations simultaneously yields (subst Cs = D·Cl):Cl = C0/(D-D·F+F)

Fractional Crystallization (Rayleigh fractionation), presuming that each infinitesimal increment of crystal growth is immediately removed from chemical contact with the melt.

take infinitesimal increments and integrate:

C=C0 F(D-1)

What if D = 1? D>1? D<1? what if melt fraction = 1?

Harker Diagram (a form of variation diagram) for olivine fractionation:Which elements are compatible in olivine and which are incompatible?

Equilibrium and fractional melting

Analogous to crystallization, except the composition of a magma changes due to varying degrees of partial melting rather than due to progressive crystallization of the magma.

Polybaric melting. Melting occurs as a mass of magma and crystals rises, decreasing P, decreasing melting T.

assimilationsurrounding rock becomes part of the magma. Requires “excess heat”, that is, it takes energy to melt rock, but if one takes energy from the magma when the magma is at the liquidus T, then it will begin to crystallize (not melt extra rock). So need to be above the liquiqus T. This can sometimes happen when a magma rises and P drops, causing the melting T to drop as well.

Liquid immiscibility

Common evolutionary trends on Earth:

Tholeiitic, lower fO2, Fe2+ = Fe concentration, often olivine early crystallizing phase

Calc-alkaline, higher fO2 (due to more H2O?), Fe3+= fe crystallizes as e.g. magnetite and is not concentrated. Also, often crystallization of pyroxene and plagioclase more than olivine.

Mineralogy and Petrology. Metamorphic Rocks, Textures, minerals, and grade, Lab #6

Consider each of the following and determine its relative metamorphic grade based on mineralogy and texture:

80: Chlorite Schist87: Silliminite garnet Gneiss77: Slate74: Phyllite (compare this also with sample R9 from the introductory lab set)

Consider each of the following, and comment on whether each is likely to be lowP-HiT (contact metamorphism), HiP-loT (dynamometamorphism), or hiP-HiT (regional metamorphism), based on texture and mineralogy

75: garnet wollastonite skarn43A: muscovite schist/gneiss86: kyanite quartzite

For each of the sample above, and also for the following samples, examine the sample, try to identify the major minerals (those included in the name of the rock), and think about metamorphic grade:

47E7224: mica garnet schist82 talc-tremolite schist83 graphite schist80 chlorite schist81 biotite gneiss (I would probably have called this a schist)93: actinolite schist95 hornblende schist

89: augen gneiss87: silliminite garnet gneiss97: hornblende gneiss

76: quartzite73: dolomite marble

T4A: granitoid gneiss (it looks like a granite, but notice the foliation)

R12 (introductory collection): gneissR8 (introductory collection): slateR12 (introductory collection): garnet, staurolite, hornblende schist/gneiss

88: cordierite, anthophyllite skarn

Thin Sections:

5: Phyllite In this sample, notice the following:foliation (the ‘layering’ is due to metamorphism, not sedimentation)The foliation is defined by alternating bands that are either quartz-rich (low interference colors)or biotite/chlorite/clay rich (darker, pleochroic, higher interference colors)

4: Schist In this sample, notice the following:large, foliated muscovite and biotite flakes (note the perfect cleavage, birdseye texture under crossed Nichols, high interference colors, biotite is strongly pleochroic)

Other minerals in this sample include quartz, feldspar (distinguished from quartz by the presence of cleavage fractures and twinning), garnet (high relief and isotropic-see below), and an opaque oxide mineral.

6: Gneiss In this sample, notice the following:Biotite and Amphibole. Both are strongly pleochroic. But amphibole shows two

directions of cleavage, not at right angles, and the crystal goes to extinction NOT PARALLEL TO THE CLEAVAGE. Biotite has only a single obvious cleavage direction and extinction IS PARALLEL TO CLEAVAGE.

At least one of the amphibole crystals has very obvious exsolution crystal in it, oriented parallel to one of the cleavage directions.

Notice that where three quartz crystals meet (look for the low-interference colors and equant shape), they tend to meet at angles that approximate 120º. This is a texture typical of metamorphic rocks. Contrast it with the round quartz crystals in sandstone, or the more irregular quartz crystals in igneous rocks.

There is some feldspar in this sample. Unlike in igneous rocks, these can’t be identified by their characteristic twinning (twins are annealed away during

metamorphism). You can distinguish it from quartz on the basis of cleavage plane lines, which the quartz lacks.

There are two “high-relief” minerals. These minerals stand out from the surrounding minerals (high-relief) because they have a distinctly higher refractive index (that is, light travels much more slowly through them, thereby refracting the light more). These two minerals look very similar in plane light, both occurring as small, equant crystals.

One of these minerals is garnet. Garnet occurs as roughly equant crystals (often hexagonal-looking), more often associated with the quartz than with biotite or amphibole, and is isotropic. Isotropic minerals are high-symmetry minerals that do not break light into separate beams, because of their high symmetry. Therefore, garnet always appears black under crossed Nichols, regardless of stage rotation.

The second high-relief minerals occurs often in association with biotite or amphibole in this thin section. It is not isotropic, but has high interference colors. I think that this mineral is clinopyroxene, although there isn’t much cleavage apparent.

44 E 7495 QuartziteIn plane light, notice the rounded outline of the original sedimentary sandstone

grains (almost all quartz). Under crossed Nichols, notice the overgrowths on the original grains, and how three grains tend to meet at 120º (this rock has not been metamorphosed as extensively as some, and the angles are not as distinctly 120º).

This is a very “clean” quartzite, with only a few rock fragments (original sand grains made up of clusters of smaller crystals) and muscovite flakes present in addition to Quartz.

44 W 6173 MarbleMost of this rock is composed of calcite. Notice the rhombohedral cleavage and

twinning pattern (cross-hatched pattern). Calcite has very high interference colors (so high that most of the color disappears and is called ‘high-order white’ – a sort of grungy gray-white). In some of the cross-hatched twins you can get hints of the high interference colors in that you can see glints or red, blue, etc. Notice the 120º intersections at the corners where three grains meet (characteristic of mono-mineralic metamorphic rocks). There are a few muscovite grains (high interference colors and birdseye under crossed Nichols, clear in plane light).

44 E 7386 Kyanite QuartziteKyanite is the high relief, medium-to-low interference-color mineral (yellowish

under crossed Nichols). It has prominent cleavage planes, and common poikiloblasts of quartz (in igneous rocks we refer to poikilocrysts – in metamorphic rocks we refer to poikiloblasts). Notice the 120º intersections between quartz grains, much better defined in this higher grade rock than in 44E7495.

Metamorphic Petrology

Metamorphism: Change in mineralogy and/or texture in the solid state

usually due to changes in T, P, or metasomatism (alteration in the presence of fluids, usually H2O, that contain various dissolved components, such as Si, Ca, Na, etc.)

T and P are not perfectly independent. Are correlated by the local geothermal gradient.

gradients in top 10000m range from about 0.015 degrees C per meter to 0.03 degrees C per meter. (or 15 to 30 degrees per kilometer)Geobarometric gradient, being caused by the weight of the overlying rock, is more regular, about 3.33 kilobars per 10000 m (or 0.333 kilobars per km). 1kbar = 100 MPascal = 0.1GPascals = 14500psi = 987 atm. (calculate how many degrees per kilobar).

burial metamorphism (up to 2kbar, 250-300C)

regional metamorphism (usually higher T, at a particular P, than typical geothermal gradient, consistent with the extra heat typical in the convergent zones where regional metamorphism occurs.

High-pressure, low T (blueschist metamorphism). lower than typical geothermal gradient

Contact metamorphism (much higher geothermal gradient, near magma intrusion.)

Shock metamorphism (coessite, stishovite, maskelynite on the Moon, other types of glass)

Boundaries of metamorphism:low end: first appearance of marker minerals (laumontite, lawsonite, albite, other

zeolites) usually at least 150-200C and 1.5kbars.high end: below partial melting, which depends on rock type and presence or

absence of water. more felsic with lots of water, melt at lower T (e.g. 650C), more mafic dry rocks melt at lower T (e.g. over 1000C)

foliation (slaty cleavage, schistosity, gneissocity)

increase in grain size (annealing, reduces surface area/volume)

triple junctions (often tend toward 120 degrees in equisized, monomineralic zones)

blasts (= lump)porphyroblasts (crystals much larger than matrix)granoblastic (equisized).

poikiloblastic (numerous small inclusions)idioblastic (well-formed crystals)hypidioblastic (medium crystal development)xenoblastic (anhedral crystals)

mylonitic (pervasive plastic deformation has caused reduction in grain size (analogous to brittly formed cataclasis at lower T and P)

blastomylonitic (e.g. augen gneiss) shear define boundaries of eye-shaped shear-bound graines.

Metamorphic Gradesee overhead or write on board if there is time (makes one think through it rather than look at it an then think one understands because one saw)

Metamorphic IsogradIsograd is a surface in three dimensions (a curve on a map) marked by the first

appearance of an index mineral as one goes from lower to higher grade metamorphism in essentially isocompositional rocks.

e.g. pelitic rocks of Highlands of Scotland, for Barrovian style metamorphism (midlin geothermal gradient). (Note increasing Ca in plag with grade)

Chlorite zone: Qz, Chloirite, Muscovite, albiteBiotite zone: Qz, Chloirite, Muscovite, albite, biotiteGarnet zone: Qz, Muscovite, biotite, garnet, Na plagStaurolite zone: Qz, Muscovite, biotite, garnet, staurolite, plagKyanite zone: Qz, Muscovite, biotite, garnet, kyanite, plag (may be staurolite)Silliminite zone: Qz, Muscovite, biotite, garnet, silliminite, plag

Isograd map of the example above. Remember that the isograds are actually surfaces.See overhead.

Metamorphic FaciesSee overhead.

Metamorphic facies is based on the idea that the mineral assemblage is at or near equilibrium and thus is a function of the T and P at which it formed and the composition of the orginal rock. Thus, if two rocks in different parts of the world formed at the same T and P, and formed from the same basic composition material, they will have the same mineral assemblages.

Metamorphic Field Gradients for various facies series superimposed on an Al2SiO5 phase diagram. Consider, which minerals have the highest density? Which the highest entropy/heat in bonds? What sequence will occur with increasing grade for different geothermal gradients? Which gradient is like the case in the Scottish Highlands? Which series will not have andalucite? Which will go from andalucite to sillimite? Which will go from kyanite to andalucite?

How are the metamorphic field gradients different from geothermal gradients? The geothermal gradient is what any particular member of the series would have experienced at its particular location in the earth during metamorphism. Another member of the series, at a different, perhaps adjacent area, might have experienced a somewhat different gradient. After these rocks are exhumed to the surface, and we examine them, we observe a series that have experienced the P-T conditions shown in this figure, but the P-T field is not necessarily the geothermal gradient present at any particular location.

Other typical minerals: cordierite at lower P series, garnet at higher P series. In general, higher density minerals at higher P, less hydrated minerals at higher T.

Petrogenetic Grid.Considers reactions more rigorously than the idea of facies. A petrogenetic grid is

a multicomponent phase diagram (thus valid only for the composition for which it is designed) showing the phase reactions in P-T space.

See overhead:

Activity: For two metamorphic field gradients, consider the sequence of reactions that occur, at what T and P particular minerals will appear. Construct an example isograd sequence based on this analysis. For one gradient, assume both increasing T and P in a roughly 45 degree angle on the diagram (say, Barrovian series). For the other gradient, assume contact metamorphism, with T increasing much faster than P (hornfels series).

Mineralogy and Petrology. Review sheet for Exam 2.Covering Igneous Petrology and Metamorphic Petrology

Igneous Rocks:

Know the important igneous textures, and what they tell us about the rock.

Be able to use the classification diagram for igneous rocks to identify a rock, or to tell about its properties given its classification.

I will have at least one hand sample to identify (name the rock), and at least one mineral to identify under microscope (name the mineral). You will only have one minute with the sample or at the microscope.

Be able to read binary phase diagrams and apply them to both textures and compositional trends that develop in igneous rocks. (non-complete examples of textures that can be explained = zoning, perthitic; be able to understand and predict how melt composition will change with crystallization for any bulk composition portrayed in a diagram.) Be able to relate the diagram to melt trends during equilibrium melting or crystallization. Know how to explain fractional crystallization using such a diagram.

Understand magma evolution. Be able to predict trends (on a graph or numerically). Be able to use the equation giving the liquid composition for any particular degree of fractional or equilibrium crystallization. Know qualitatively what fractional or equilibrium melting are, and what assimilation is.

Know about the cal-calkaline and tholeiitic trends. Know their chemistry, the process that produces chemical changes, and the geological environments where each is more common.

Metamorphic Rocks:

Know the important metamorphic textures.

Understand the concepts of geothermal gradient, metamorphic grade, isograd, metamorphic facies. Be able to explain them, what they are, etc.

Be able to map the isograds (based on mineralogy) for a pelitic assemblage of the Barrovian series, or list the minerals in each isograd if the isograds are given to you.

Be able to read and interpret a petrogenetic grid!!!!

I will have at least one hand sample to identify (name the rock), and at least one mineral to identify under microscope (name the mineral). You will only have one minute with the sample or at the microscope.

Final Exam

Comprehensive, open notes.

85% of questions will be the SAME TYPE (that is analogous questions) to what was on the regular exams.

15% of the questions will be material on the previous review sheets but not specifically covered on the exams.