Chapter 4Rocks and Minerals:

Documents that Record Earth's History

What can Minerals Tell Us?1. Minerals may contain radioactive elements that can be used

for radiometric age dating. 2. Minerals that crystallize from magmas and lavas can provide

information about temperatures, as well as viscosity of the magma, type of volcano, and tectonic setting.

3. Minerals that form under metamorphic conditions can provide information about temperatures and pressures, from which we can determine the depth at which metamorphism occurred, and information about the history of the formation of mountain ranges.

4. Minerals that form by evaporation in arid climates can tell us about paleoclimatic conditions. Since some climates are controlled by latitude, we can make general inferences about latitude.

5. Minerals that form in sea water tell us about the nature of ancient seas.

6. Minerals which contain iron can record the orientation of the Earth's magnetic field, which yields information on latitude, and provides evidence for drifting continents, sea floor spreading, and movement and reversal of the Earth's magnetic poles.

7. Minerals in sedimentary rocks can provide information on the tectonic setting, amount of relief, paleoclimate, and types of rocks that are eroding in the source area.

8. Minerals can also tell us about the changing chemistry of the atmosphere, for example, the presence or absence of oxygen.

What can Minerals Tell Us?

Minerals By definition, minerals are:

1. Naturally occurring 2. Inorganic 3. Solid 4. Definite chemical composition 5. Orderly internal crystal structure

Each mineral has different physical and chemical properties, which make it easy for us to identify the different species of minerals.

Minerals

Some Physical Properties of Minerals• color • streak • luster • hardness • density • crystal form • cleavage

• fracture • magnetism • reaction to acid • taste • flexibility • feel

Physical Properties of Minerals • Color - the color or range of colors of a mineral as it

appears to the eye in reflected light.

• Examples: – Quartz may be colorless, white, pink, purple, dark brown, green or blue. – Pyrite is always gold.

Physical Properties of Minerals• Streak - the color of a mineral when it is ground to a

powder. Streak color may be quite different from the whole mineral color.

• Examples: – Hematite may be silver or gray, but it has a reddish brown streak. – Pyrite is gold, but is has a black streak.

Physical Properties of Minerals• Luster - the character of the light reflected from the

mineral. A mineral may have a metallic luster or a non-metallic luster.

Physical Properties of Minerals• Hardness - the resistance of a mineral to scratching. • Hardness is measured on a scale of 1 - 10 called Mohs

Hardness Scale. • Hardness of minerals can also be compared to common

objects (fingernail, copper penny, nail, glass).

Mohs Hardness ScaleTalc (softest) Gypsum← fingernailCalcite← penny (copper)Fluorite← nailApatite← glass Orthoclase feldspar (potassium feldspar) Quartz Topaz Corundum Diamond (hardest)

Physical Properties of Minerals• Density - how heavy a mineral is for its size. • The mass of a mineral divided by its volume is a measure

of its density. • Examples:

– Quartz has a density of 2.65 g/cm3. – Gold has a density of 19.3 g/cm3.

Physical Properties of Minerals• Crystal form - some minerals are in the form of crystals. Crystal shape is

related to the structural arrangement of atoms within the mineral. • Crystals enlarge through addition of ions to their surfaces as they crystallize. • Perfect crystals are rare because minerals typically grow close together in

confined spaces, producing a mass of interlocking crystals. • A crystal which form in a large space may develop perfect crystal faces.

Physical Properties of Minerals• Cleavage - the tendency of a mineral to break along flat surfaces related to

planes of weakness in its crystal structure. Minerals can be identified by the number of cleavage planes they exhibit, and the angles between them.

• Examples: – Some minerals tend to cleave or break into flat sheets (the micas: muscovite and

biotite). – Others break into cubes (halite), or into rhombs (calcite and dolomite).

Physical Properties of Minerals• Fracture - irregular breakage, not related to planes of

weakness in the mineral.

• Some minerals, such as quartz and olivine, do not have cleavage. They have a type of fracture called conchoidal fracture. Conchoidal fracture produces curved breakage surfaces, as seen on arrowheads or chipped glass.

Physical Properties of Minerals• Magnetism - A few minerals are magnetic. They are

attracted to a magnet, or they act as a natural magnet, attracting small steel objects such as paperclips.

• Example: – Magnetite.

Physical Properties of Minerals• Reaction to acid - The carbonate minerals react with

diluted hydrochloric acid (HCl) by effervescing or fizzing, producing bubbles of carbon dioxide gas.

• Examples: – Calcite fizzes readily in hydrochloric acid. – Dolomite will fizz if it is first scratched and powdered.

Physical Properties of Minerals• Taste - Some minerals have a distinctive taste.

• Example: – Halite has a salty taste. It is used as table salt.

Physical Properties of Minerals• Flexibility - Some minerals can be bent.

• Examples: – Muscovite and biotite mica are elastic. When bent they return to their

original shape. – Gypsum is flexible. It bends and stays bent.

Physical Properties of Minerals• Feel - Some minerals have a distinctive feel to the fingers.

• Example: – Talc has a soapy feel.

Rock-Forming Minerals • There are more than 3000 minerals on the Earth, but only

a few are common and make up most of the rocks.

• The common rock-forming minerals can be divided into two groups:– Silicates – Non-silicates.

Silicate Minerals• Earth's crust is dominated by 2 chemical elements:

– Oxygen (46.6% by weight) – Silicon (27.7% by weight)

– These elements help make up the dominant group of rock-forming minerals, the silicate minerals.

• Examples: quartz, feldspar, mica

Silicate Minerals - Structure• The silicate minerals are based on a crystal structure that

involves four oxygen atoms arranged in pyramid-like shape, surrounding a smaller silicon atom.

• This structure is called the silicate tetrahedron.

Silicate Minerals - Feldspar • Dominant mineral in Earth's crust.• Two directions of cleavage at 90o

• Flat, glassy rectangular surfaces. • Color may be white, pink, gray, green.• Common in igneous rocks such as granite and basalt.

Silicate Minerals - FeldsparTwo major types: • Orthoclase (potassium feldspar) - KAlSi3O8

• Plagioclase - A range of compositions with sodium and calcium. – Calcium-rich = anorthite (CaAl2Si2O8)

– Sodium-rich = albite (NaAlSi3O8)

Silicate Minerals - Quartz• Second-most abundant mineral in Earth's crust.• Color varies - colorless, white (milky quartz), gray to brown (smoky quartz),

pink (rose quartz), purple (amethyst), blue, or green. • Hard (scratches glass) • Glassy luster • Conchoidal fracture. • Six-sided, elongated crystals.

Silicate Minerals - Quartz• Common in granite• Resists weathering; common in some sands in humid areas• Major constituent of quartz sandstone and quartzite. • Chert is composed of microcrystalline quartz.

Silicate Minerals - Mica• Perfect cleavage in one direction causing it to

split into thin sheets.• Two types:

– Muscovite - Colorless or silver-colored mica.

– Biotite - Black or dark brown mica (contains Mg and Fe).

Silicate Minerals - Amphiboles• Two directions of cleavage, not at 90o

• Narrow, elongated crystals• Typically dark in color (black or dark green). • Common in metamorphic rock amphibolite. • Example: Hornblende. Contains Mg and Fe.

Silicate Minerals - Olivine • Olive green color• Glassy texture. • No cleavage. • Conchoidal fracture. • Contains Mg and Fe.• Main constituent of the ultramafic rock,

peridotite (birthstone = peridot).

Silicate Minerals - Clays • Group of minerals formed from

weathering of feldspars and some other minerals.

• Very fine-grained• Dull, earthy luster • Soft, smooth feelExample: Kaolinite, a white clay with many

economic uses.

Non-silicate Minerals

– carbonates – halides– phosphates, etc.

Types:– native elements– oxides – sulfides – sulfates

Non-silicate minerals comprise about 8% of the Earth's crust. Carbonate minerals are the most widespread.

Carbonate minerals • Calcium carbonate.

– Calcite (CaCO3)

– Aragonite (CaCO3)

• Calcium magnesium carbonate. – Dolomite (CaMg(CO3)2)

Calcite • Main constituent of limestone and marble. • Shells of some marine organisms.• Fizzes in hydrochloric acid. • Has rhombohedral cleavage (three directions not at 90o). Cleavage fragments are rhombs.

Aragonite • Same chemical formula as calcite, but it has a different

crystal structure.• Fizzes in hydrochloric acid.• Shells and skeletons of corals and mollusks (clams and

snails).

Dolomite• Has rhombohedral cleavage like calcite.• Will fizz in acid only when scratched or powdered.• Main constituent of sedimentary rock dolostone or

dolomite.• Forms from alteration of limestone through the addition of

Mg.

Evaporite minerals • Halite (NaCl) • Gypsum (CaSO4 . 2H2O)

• Anhydrite (CaSO4)

Halite • Major constituent of rock salt

(and table salt).• Cubic cleavage • Salty taste.• Typically colorless to white or

pink.

Gypsum • Major constituent of rock gypsum.• Used in Plaster of Paris and drywall. • Soft - can be scratched by fingernail. • Typically white or colorless to pink. • Varieties:

– Selenite - clear crystals with rhombohedral cleavage

– Alabaster - fine-grained and massive – Satin spar - fibrous

Rocks • A rock is an aggregate of one or more minerals. • Rocks are the building blocks of the Earth's crust.

Rocks 3 Types1. Igneous - Crystallized from hot, molten rock.

Examples: granite, basalt 2. Sedimentary - Fragments of sediment laid down by

water or wind become compressed or cemented over time Examples: sandstone, shale, limestone

3. Metamorphic - Rocks changed by heat and/or pressure or chemical activityExamples: gneiss, schist, slate, marble

The Rock Cycle Through the rock cycle,

one type of rock can be converted into another.

Igneous Rocks • The word igneous means "fire-formed."• Igneous rocks crystallized from hot, molten

magma or lava, as it cooled. – Magma is hot, molten rock beneath the surface

of the Earth.– Lava is hot, molten rock which has flowed out

on the surface of the Earth. • Igneous rocks make up more than 90% of Earth's

crust, by volume.

Extrusive Igneous RocksExtrusive or volcanic rocks form from lava, which

cooled on the Earth's surface.

Examples: Basalt, rhyolite, andesite, obsidian