Wk. 8 Minerals and Rocks: Formation, identification and ...€¦ · Mineral Properties • Mineral...
Transcript of Wk. 8 Minerals and Rocks: Formation, identification and ...€¦ · Mineral Properties • Mineral...
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Wk. 8
Minerals and Rocks:
Formation, identification and classification
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Earth Materials – Minerals
• Gemstones and other minerals, such as gold, have fascinated people for thousands of years and have been supposed to have “mystical” or “curative” powers.
• Minerals have many essential uses in industrial societies.
• Minerals are the basic units that make up most of Earth’s materials.
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• What are rocks?
– are solid aggregates of minerals.
• Rocks find many uses...
– rocks crushed for aggregate in cement and for roadbeds.
– sawed and polished rocks for tombstones,
monuments, mantle pieces and counter tops
– Even the soils we depend on
• for most of our food
• formed by alteration of rocks
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What are Minerals?
– naturally occurring.
– crystalline solid.
crystalline means that minerals have an ordered internal arrangement of their atoms.
– minerals have a narrowly defined chemical composition.
– and characteristic physical properties such as
density
hardness
color...
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Matter and its Composition
• Matter
– is anything that has mass and occupies space.
– exists as solids, liquids, and gases.
– consist of atoms.
• Element
– is a chemical substance.
– cannot be chemically decomposed into simpler
substances.
– is composed of tiny particles called atoms.
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What are Atoms?
• Atoms are the smallest units of matter
retain the characteristics of the element
• Atoms have
a compact nucleus containing
• protons – particles with a positive electrical charge
• neutrons – electrically neutral particles
particles orbiting the nucleus
• electrons – negatively charged particles
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Structure of an Atom
• The dense nucleus of an atom
– consisting of protons and
neutrons
– is surrounded by a cloud of
orbiting electrons.
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Structure of an Atom:
Example Sodium (Na)
Na23
11
Nucleus: 11 protons +
12 neutrons.
Cloud: 11 electrons
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Atoms: Atomic number & Atomic mass number
• Atomic number:
= the number of protons
• Atomic mass number:
= number of protons + number of neutrons
The number of neutrons in an atom may vary ...
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Isotopes
• The different forms of an element’s atoms with varying
numbers of neutrons are called isotopes.
• Different isotopes of the same element
have different atomic mass numbers
behave the same chemically
• Isotopes are important in radiometric dating.
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Example: Carbon Isotopes
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Electrons and Shells
• Electrons orbit the nucleus in one or more shells.
• The outermost shell participates
in chemical bonding
contains up to 8 electrons.
• Noble gas configuration of 8 electrons
or 2 for Helium.
completes the outermost shell.
• Other atoms attain a noble gas configuration in the process of bonding.
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Bonding and Compounds
• Bonding:
the process whereby atoms join to other atoms.
• Compound:
a substance resulting from the bonding of two or
more elements.
Example:
Oxygen (O2) is an element.
Ice is a compound made up of hydrogen and oxygen
(H2O).
• Most minerals are compounds.
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Ionic Bonding
• Ion– atom that has gained or lost one or more electrons
negative or positive charge.
• One way for atoms to attain the noble gas configuration is by transferring electrons
producing ions
• Ionic bonding attraction between two ions of opposite charge.
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Covalent Bonding
• Another way for atoms to attain the noble gas
configuration is by sharing electrons:
• Covalent bonding
results from sharingelectrons
shared electrons
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Minerals—The Building Blocks of Rocks
• A mineral’s composition is shown by a chemical
formula
a shorthand way of indicating how many atoms of
different kinds it contains.
Quartz: SiO2
Ratio: 1: 2
– Quartz consists of 1 silicon atom for every 2 oxygen atoms
– Orthoclase consists of 1 potassium, 1 aluminum, and 3 silicon for every 8 oxygen atoms
KAlSi3O8
1: 1: 3: 8
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Native Elements
• A few minerals consist of only one element.
• They are not compounds.
• They are known as nativeelements.
• Examples:
gold – formula: Au
diamond – formula: C
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Crystalline Solids
• By definition, minerals are crystalline solids
– with atoms arranged in a specific 3D framework.
• If given enough room to grow freely,
minerals form perfect crystals with
planar surfaces, called crystal faces
sharp corners
straight edges.
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Narrowly Defined Chemical Composition
• Some minerals have very specific compositions
– examples are halite (NaCl) or quartz (SiO2).
• but others have a range of compositions
because one element can substitute for another if the atoms of the two elements have
• the same electrical charge
• and are about the same size
Example: olivine
• (Mg,Fe)2SiO4
• iron and magnesium substitution in any proportion
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Mineral Properties
• Mineral properties are controlled by:
Chemical composition
Crystalline structure
• Mineral properties are particularly useful
for mineral identification and include:
• cleavage
• fracture
• hardness
• specific gravity
• color
• streak
• luster
• crystal form
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How Many Minerals Are There?
• More than 3500 minerals are known.
• Only about 2 dozen are particularly common.
• Many others are important resources.
• Mineral groups:
– minerals with the same negatively charged ion or ion
group belong to the same mineral group.
• Most minerals in the crust belong to the group called
silicates.
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Silicates
• Silicates are minerals containing silica
Si and O
• They make up perhaps 95% of Earth’s crust and
account for about 1/3 of all known minerals.
• The basic building block of silicates is the silica
tetrahedron
• which consists of one silicon atom
• surrounded by four oxygen atoms
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Types of Silicates
• Silica tetrahedra can be
– isolated units bonded to other
elements
– arranged in chains (single or
double)
– arranged in sheets
– arranged in complex
3D networks
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Types of Silicates
• Ferromagnesian silicates
contain iron (Fe), magnesium (Mg), or both.
• Nonferromagnesian silicates
do not contain iron or magnesium
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Ferromagnesian Silicates augite, a pyroxene.
Hornblende, an
amphibole.
biotite mica
olivine
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Nonferromagnesian Silicates
Quartz Potassium feldspar
Plagioclase feldspar Muscovite
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Other Mineral Groups
• Carbonates
– minerals with carbonate ion (CO3)-2
calcite (CaCO3),
• found in limestone
dolomite [CaMg(CO3)2],
• found in dolostone
• Other mineral groups are important, but more as
resources than as constituents of rocks.
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Rock-Forming Minerals
• Most rocks are solid aggregates of one or more minerals.
• Thousands of minerals occur in rocks, but only a few are
common
and called rock-forming minerals.
• Most rock-forming minerals are silicates, but carbonates are
also important.
• Accessory minerals are present in small amounts and are
ignored in classifying rocks.
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―The Rock Cycle‖
• The rock cycle is a pictorial representation of events
leading to
the origin, destruction, change
and reformation of rocks
• Rocks belong to 3 major families
– igneous
– sedimentary
– metamorphic
• The rock cycle shows how these rock families are
interrelated and can be derived from one another.
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Pyroclastic
material
Lava
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Igneous Rocks
• All igneous rocks
cool and crystallize from magma,
solidify from lava,
or consolidate from pyroclastic materials.
• Magma is molten material, below the surface.
• Lava is molten material on the surface
• Pyroclastic materials are particles such as volcanic ash
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Pyroclastic
material
Lava
Igneous Part of the Rock Cycle
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Categories of Igneous Rocks:
• Extrusive or volcanic rocks
– formed at the surface
– from lava or pyroclastic materials
• Intrusive or plutonic rocks
– formed from magma injected into the crust
– or formed in place in the crust
• Plutons are intrusive bodies
consisting of plutonic rock
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Plutons
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Igneous Rock Textures:
• Texture
– is the size, shape and arrangement of crystals, grains and other constituents of a rock.
• Igneous rocks have 4 different textures
relate to cooling rate of magma or lava.
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4 Cooling-Rate Textures:
• phaneritic,
– with visible grains
• cooled slowly
• aphanitic,
– with grains too small to see without magnification
• cooled quickly
• porphyritic,
– with larger grains surrounded by a finer-grained groundmass
• cooled slowly first, then more quickly
• glassy,
– with no grains
• cooled too quickly for minerals to grow
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Igneous Rock Textures:
Other textures reveal further details of the formation of the
rock:
• Vesicular texture, with holes (vesicles),
– indicates the rock formed as water vapor and other
gases became trapped during cooling of lava.
• Pyroclastic or fragmental texture,
– containing fragments,
– formed by consolidation of volcanic ash
– or other pyroclastic material.
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Igneous Rock Textures:
Rapid cooling
Aphanitic
texture
Slow cooling
Phaneritic
texture
2-stage cooling
Porphyritic
texture
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Igneous Rock Textures:
Glassy texture
cooling was too
rapid for
mineral growth
Vesicular texture
gasses trapped in
cooling lava
Pyroclastic texture
particles fragmented
during eruption
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Classifying Igneous Rocks:
• Texture and composition are the criteria used to classify
most igneous rocks.
• Composition categories are based on silica content:
felsic (>65% silica)
intermediate (53-65% silica)
mafic (45-52% silica)
• More felsic magmas have higher Na, K, Al.
• More mafic magmas have higher Ca, Fe, Mg.
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Common Igneous Rocks:
Basalt Gabbro
Andesite Diorite
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Common Igneous Rocks:
RhyoliteGranite
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Classifying Igneous Rocks with Special Textures:T
extu
re
Vesicular
Glassy
Pyroclastic or
Fragmental
Composition
Pumice
Obsidian
Volcanic breccia
Tuff/welded tuff
MaficFelsic
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Igneous Rocks with Special Textures:
Tuff has pyroclastic texture.Pumice is glassy and
extremely vesicular.
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Sedimentary Rocks:
• Sedimentary rocks form by the lithification of sediment
• In the rock cycle, sediment originates when:
– mechanical and chemical weathering
• breaks rocks down into smaller particles
• and into solution
Transport removes sediment
• from its source area
• and carries it elsewhere
– Running water, glaciers, wind and waves
• transport sediment
Deposition involves settling of particles,
• and chemical and biological extraction of minerals from solution
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Sedimentary Part of the Rock Cycle
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• Lithification means turning loose sediment into rock
• Lithification occurs by burial
• when additional sediment accumulates on top
compaction
• reduction of the amount of pore space between particles
• because of the weight of overlying sediment
cementation
• precipitation of minerals within pores
• that effectively binds sediment together
– calcium carbonate (CaCO3) cement is common
– silica (SiO2) cement is common
– iron oxide (Fe2O3) cement is less common
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Categories of Sedimentary Rocks:
• Detrital sedimentary rocks
– consist of solid particles
– derived from preexisting rocks (detritus)
• Chemical sedimentary rocks
– consist of minerals derived from materials in solution and extracted by either
• inorganic chemical processes
• or by the activities of organisms
– subcategory biochemical sedimentary rocks, for which
• the activities of organisms are important.
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Detrital Sedimentary Rocks:
• are composed of fragments or particles
known as “clasts” = Clastic texture
• These rocks are defined primarily by the size of clasts
• conglomerate
– composed of gravel (>2mm)
– rounded clasts
• sedimentary breccia
– also composed of gravel (>2mm)
– but clasts are angular
• sandstone
– composed of sand
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• Mudrocks consist of particles < 1/16 mm
– siltstone
• composed of silt-sized particles - 1/16-1/256 mm,
• feel slightly gritty,
• but not visible without magnification
– mudstone
• composed of a mixture of silt- and clay-sized particles
– claystone
• composed of clay-sized particles
– <1/256 mm, feel smooth even to the teeth
– shale
• mudstone or claystone that
• breaks along closely spaced parallel planes (fissile)
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Chemical Sedimentary Rocks:
• Recall that these rocks result
– when inorganic chemical processes
– or organisms extract minerals from solution
• This can result in different textures
– Crystalline texture
• has an interlocking mosaic of mineral crystals
• results from chemical precipitation
– Clastic texture
• has an accumulation of broken pieces of shells
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Chemical Sedimentary Rocks:
• Limestone – carbonate rock made of calcite precipitated chemically or by organisms
• Dolostone – carbonate rock made of dolomite usually formed from limestone
• Evaporites formed by
inorganic chemical precipitation during evaporation
– Rock salt – evaporite made of halite
– Rock gypsum – evaporite made of gypsum
• Chert – compact, hard, fine grained silica, formed by chemical or biological precipitation (some consisting of microscopic shells of silica-secreting organisms)
• Coal – made of partially altered, compressed remains of land plants accumulated in swamps
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Common Sedimentary Rocks:
ShaleQuartz sandstone
Sedimentary brecciaConglomerate
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Common Sedimentary Rocks:
Rock salt
Rock gypsum
Fossiliferous limestone
Chert
Coal
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Metamorphic Rocks:
• Metamorphic rocks
– result from transformation of other rocks
– in the solid state, without melting
• Changes resulting from metamorphism:
compositional:
• new minerals form
textural:
• minerals become reoriented
• minerals recrystallize
– or both.
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Earth StoryMetamorphic Part of the Rock Cycle
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Agents of Metamorphism:• Heat provides new conditions
– where different minerals may be stable – and increases the rate of chemical reactions
• Pressure– Lithostatic pressure provides new conditions
• where different minerals may be stable • and forms smaller denser minerals
– Differential pressure• exerts force more intensely from one direction • causing deformation • and development of foliation.
• Fluid activity enhances metamorphism – by increasing the rate of chemical reactions – by transporting ions in solution
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Types of Metamorphism:
• Contact metamorphism
– heat
– chemical fluids
– from an igneous body
– alter rocks adjacent to the magma
• Regional metamorphism
– large, elongated area
– tremendous pressure
– elevated temperatures
– fluid activity
– occurs at convergent and divergent plate boundaries
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Metamorphic Textures:
• Foliated texture
– platy and elongate minerals aligned parallel to one
another
– caused by differential pressure
• Nonfoliated texture
– mosaic of roughly equidimensional minerals
– or platy and elongate minerals
– arranged in a helter-skelter fashion
– with random orientations
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Formation of Foliation:
• When rocks are subjected to differential pressure
minerals typically rearrange or grow parallel to each other
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Formation of Foliation:
Microscopic view of a
metamorphic rock
with foliation
showing the parallel
arrangement of
minerals
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Foliated Metamorphic Rocks:
• Slate
– very fine-grained, breaks in flat pieces
• Phyllite
– fine-grained (coarser than slate but grains are still
too small to see without magnification)
– breaks in flat pieces
• Schist
– clearly visible platy and/or elongate minerals
• Gneiss
– alternating dark and light bands of minerals
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Nonfoliated Metamorphic Rocks:
• Marble
– made of calcite or dolomite from limestone or dolostone
• Quartzite
– made of quartz from quartz sandstone
• Greenstone
– made of green mafic igneous rock
• Hornfels
– results from contact metamorphism
• Anthracite
– made of black lustrous carbon from coal
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Common Metamorphic Rocks:
Slate Schist
Gneiss Marble Quartzite
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Plate Tectonics and the Rock Cycle:
• The atmosphere, hydrosphere and biosphere act on earth materials and cause weathering erosion and deposition
• Earth’s internal heat aids melting and metamorphism
• Plate tectonics recycles Earth materials
– heat and pressure at convergent plate boundaries
• lead to metamorphism and igneous activity
– resulting deformation makes mountains
• that in turn weather and erode to form sediment
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End of Lecture