rocks and minerals: documents that record earth's...

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.

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.


• 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.


• Luster - the character of the light reflected from the mineral. A mineral may have a metallic luster or a non-metallic luster.


• 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 Scale1. Talc (softest)

2. Gypsum← fingernail

3. Calcite← penny (copper)

4. Fluorite← nail

5. Apatite← glass

6. Orthoclase feldspar (or potassium feldspar)

7. Quartz 8. Topaz 9. Corundum 10. Diamond (hardest)


• 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.


• 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.


• 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).


• 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.


• 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.


• 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.


• Taste - Some minerals have a distinctive taste.

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


• 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.


• 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 - Feldspar

Two 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

1. 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 Rocks

Extrusive or volcanic rocks form from lava, which cooled on the Earth's surface.

Examples: Basalt, rhyolite, andesite, obsidian

Intrusive Igneous Rocks

Intrusive or plutonic igneous rocks form from magma which cooled beneath the surface of the Earth.

• Examples: Granite, gabbro, diorite

Cooling History and Grain Size

• The texture of a rock is a description of its grain size. • Cooling rates influence the texture of the igneous rock. • Lava cools much more quickly than magma because lava

is on the surface of the Earth, where temperatures are much lower than they are at depth.

Cooling History and Grain Size

Extrusive rocks = quick cooling = fine grained Intrusive rocks = slow cooling = coarse grained

Extrusive vs. Intrusive

Rhyolite - fine-grained, extrusive igneous rock.

Granite - coarse-grained, intrusive igneous rock.

Igneous Rock Classification

Igneous rocks are classified on the basis of:

1. Texture (or grain size) 2. Composition

Igneous Rock Composition Groups

1. Silica-rich2. Intermediate3. Silica-poor

Silica-rich Rocks1. High percent silica. 2. Light-colored. 3. Has light-colored minerals such as quartz and potassium

feldspar.Examples: granite, rhyolite.

Intermediate Rocks1. Intermediate in composition between silica-rich and

silica-poor. 2. Mixture of light and dark minerals. Examples: diorite, andesite.

Silica-poor Rocks1. Iron and magnesium rich. 2. Dark-colored. 3. Has dark minerals such as olivine, pyroxene, and

amphibole. Examples: gabbro, basalt.

Very silica-poor Rocks1. Very iron and magnesium rich. 2. Typically green in color due to abundant olivine. Example: Peridotite.

Igneous Rock Classification Silica-rich (silicic) Intermediate Silica-poor

(mafic)Very silicapoor(ultramafic)

Fine

Rhyolite Andesite Basalt

Coarse

Granite Diorite Gabbro Peridotite

Basalt

• The most common igneous rock.• Ocean crust is dominated by basalt. Covers about 70% of

Earth's surface. • Islands like Hawaii and Iceland are made of basalt. • Fine-grained texture • Dark color because it contains ferromagnesian (Fe and Mg)

minerals, along with feldspar.

Granite

• Earth's continental crust is dominated by granite. • Coarse-grained texture.• Light color because it is dominated by light-colored

minerals like quartz and feldspar.

Bowen's Reaction Series

Minerals in igneous rocks crystallize in a particular order, at particular temperatures.

Sedimentary Rocks

• Cover about 75% of the world's land area.

• Form when loose sediment (gravel, sand, silt or clay) becomes compacted and/or cemented to form rock.

• The process of converting sediment to sedimentary rock is called lithification.

Sediment is deposited in horizontal layers. A major characteristic of sedimentary rock is layering, also called bedding or strata.

Sedimentary Rocks

Sedimentary rocks contain the fossil record, which preserves the evolving story of life on Earth.

Sedimentary Rocks

What can sedimentary rocks tell us?

• Locations of ancient sedimentary environments (seas, reefs, deltas, beaches, rivers, lakes deserts, glaciers, and mountains).

• Ancient climates– humid tropical coal swamps, – dry windswept deserts, – glacial ice sheets, – high temperatures and high sea levels.

Sedimentary rocks also hold the fossil fuels and energy resources on which our culture depends - coal, oil, natural gas. Careful reading of the rock record allows exploration geologists to find these critical resources.

Sedimentary Rocks

How is sediment formed?

Sediment forms from the weathering and erosion of rocks, as part of the rock cycle.

Weathering of granite in a humid climate

1. Feldspars undergo hydrolysis to form clay. 2. Biotite and amphibole undergo hydrolysis to form clay, and oxidation to

form iron oxides.3. Na, Ca, and K ions are lost in solution and washed away. 4. Small amounts of Si from feldspars, biotite, and amphibole are lost in

solution. 5. Quartz remains as sand grains due to its resistance to weathering.

Fate of the Weathering Products

• Clay minerals form shale• Iron oxides form cement, ochre, or iron ore• Dissolved Na, Ca, and K ions form limestone, evaporites,

or become included in shale• Dissolved Si ions form chert, silica cement, or diatomite• Unaltered quartz grains form sandstone

Types of Sedimentary Rocks

• Clastic Sedimentary Rocks (also called terrigenous or detrital)

• Chemical / biochemical Sedimentary Rocks

• Organic Sedimentary Rocks (Coal)


1. Clastic sedimentary rocks (also called terrigenous or detrital)

– Conglomerate or Breccia – Sandstone – Siltstone – Shale or Claystone

2. Chemical/biochemical sedimentary rocks 3. Organic sedimentary rocks (coal)



2. Chemical/biochemical sedimentary rocks– Evaporites – Carbonate sedimentary rocks (limestone and dolostone or dolomite) – Siliceous sedimentary rocks (chert, diatomite)

3. Organic sedimentary rocks



2. Chemical/biochemical sedimentary rocks3. Organic sedimentary rocks

– Peat – Lignite– Bituminous coal – Anthracite coal

Clastic Sedimentary Rocks

Clastic sedimentary rocks are derived from the weathering of pre-existing rocks, which have been transported to the depositional basin.

Clastic Texture

• Clasts (larger pieces, such as sand or gravel) • Matrix (mud or fine-grained sediment surrounding the clasts)

• Cement (the chemical "glue" that holds it all together) Types of cement:

• Calcite • Iron oxide • Silica

Clastic Sedimentary Rocks are Classified by Grain Size

• Gravel - Grain size greater than 2 mm • Sand - Grain size 1/16 to 2 mm • Silt - Grain size 1/256 to 1/16 mm • Clay - Grain size less than 1/256 mm

Clastic Sedimentary Rocks are classified by grain size

Grain size Rock name

Gravel Conglomerate = rounded clasts Breccia = angular clasts

Sand Sandstone

Silt Siltstone

Clay Shale = fissile Claystone = massive

Chemical/Biochemical Sedimentary Rocks

Form within the depositional basin from chemical components dissolved in the seawater.

Chemicals are removed from seawater and made into rocks by chemical processes, or biological processes (such as shell growth).

Chemical/Biochemical Sedimentary Rocks

1. Evaporites - form from the evaporation of water 2. Carbonate rocks - form by chemical processes and

biochemical processes 3. Siliceous rocks - form from chemical processes (silica

replacing limestone) or biochemical processes (silica-secreting organisms)

Evaporites

1. Rock salt - composed of halite (NaCl). 2. Rock gypsum - composed of gypsum (CaSO4 . 2H2O)

3. Travertine - composed of calcium carbonate (CaCO3) – a carbonate rock; forms in caves and around hot springs.

Carbonate Rocks 1. Limestones

– Micrite (microcrystalline limestone) – Oolitic limestone – Fossiliferous limestone – Coquina – Chalk – Crystalline limestone – Others

2. Dolostones or dolomites

Siliceous rocks • Diatomite - made of microscopic planktonic organisms

called diatoms. Resembles chalk, but does not fizz in acid.• Chert - massive and hard, microcrystalline quartz. May be

dark or light in color. Often replaces limestone. Does not fizz in acid.

Organic Sedimentary Rocks - Coal

Composed of organic matter (plant fragments). With increasing depth of burial (temperature and pressure):

• Peat • Lignite • Bituminous coal • Anthracite coal

Organic Sedimentary Rocks - Coal

• Coal is a fossil fuel. Electric utility companies use more than 90% of the coal mined in the U.S.

• Chemicals derived from coal are used in making plastics, tar, synthetic fibers, fertilizers, and medicines.

Metamorphic Rocks

• Metamorphic means "changed form."• Metamorphism causes changes in the texture and

mineralogy of rocks.• Rocks are changed or metamorphosed by:

1. High temperatures 2. High pressures 3. Chemical reactions caused by solutions and hot gases

Types of Metamorphism

1. Contact metamorphism Alteration of rock by heat adjacent to hot molten lava or

magma.2. Regional metamorphism Alteration of rock over a large area by heat and pressure

due to deep burial or tectonic processes.

Types of Metamorphic Rocks

Metamorphic rocks are separated into two groups on the basis of texture. • Foliated• Non-foliated (or granular)

Foliation = Laminated structure in a metamorphic rock resulting from the parallel alignment of sheet-like minerals (usually micas).

Foliated Metamorphic Rocks

In order of increasing grade of metamorphism:• Slate• Phyllite • Schist • Gneiss


Slate - Mica flakes are microscopic in size. Derived from the regional metamorphism of shale. Note the relict sedimentary bedding (vertical).

Phyllite - Mica flakes are very fine-grained; other minerals such as garnet or staurolite may also be present. Derived from the regional metamorphism of shale.


Schist - Mica flakes are visible to the unaided eye. Derived from the regional metamorphism of shales or fine-grained volcanic rocks.


Gneiss - Coarse-grained rock with minerals segregated into light and dark layers or bands. Derived from the regional metamorphism of high-silica igneous rocks, and muddy sandstones.

Non-foliated Metamorphic Rocks

Non-foliated or granular metamorphic rocks are composed of equidimensional grains such as quartz or calcite. There is no preferred orientation. The grains form a mosaic.


Marble - Composed of finely- to coarsely-crystalline calcite or dolomite. Derived from the metamorphism of limestone or dolostone. Commonly white or gray. May be pink.


Greenstone - Contains iron- and magnesium-rich green minerals such as chlorite and epidote. Fine-grained texture. Derived from the low-grade metamorphism of basalt.


Hornfels - Very hard, fine-grained rock. Derived from the contact metamorphism of shale and other fine-grained rocks.

Metamorphic Index Minerals

Certain minerals form during metamorphism, under specific pressure and temperature conditions. These minerals can be used as a guide to metamorphic pressures and temperatures. They are called metamorphic index minerals.


• Chlorite and muscovite form at relatively low temperatures.

• Biotite and garnet form at somewhat higher temperatures and pressures.

• Staurolite and kyanite form at intermediate to high temperatures and pressures.

• Sillimanite forms at the highest temperatures and pressures.