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10/3/2019

Rocks and Minerals

Chapter 7

This lecture will help you understand:

1. Earth Scientists: Nature Detectives 2. Elements and Atoms: Basic Building Blocks 3. Minerals 4. Igneous Rocks 5. Sedimentary Rocks 6. Metamorphic Rocks 7. The Rock Cycle and Mineral Resources

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Understanding rocks enables scientists to

• Locate mineral resources (e.g., copper, gypsum) • Find fossil fuels (e.g., oil, gas, coal) • Assess the risk from natural hazards such as volcanic eruptions and

tsunami • Learn about Earth processes such as plate tectonics • Discover the history and origins of other planets

Original ideas about how rocks formed

Neptunism • Rocks formed in a global ocean when material sank to ocean floor or was

precipitated from chemical reactions Plutonism • Heat from Earth’s interior melted rocks or caused them to fuse together

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Materials produced from natural Resources

• Bricks are made from raw materials such as shale or fireclay found at Earth’s surface

Where do bricks come from?

Raw materials smashed into smaller pieces in crusher. Crushed materials ground to smaller size by grinding wheel. Resulting particles passed through a series of screens to sort materials by size. Sugar- and flour-sized particles mixed with water and other ingredients. Small particles mixed with water and other ingredients.

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Bricks Wet mixture forced through brick-shaped form. Wet “bar” cut into smaller brick-sized pieces. Excess water removed by passing wet bricks slowly through long dryers (200oC). Final stage is “firing” of bricks in kiln at high temperatures (1,100oC).

Rocks are made of minerals

• ~20 common minerals • Example: The rock granite (below) is composed of 4 key minerals -

feldspar, quartz, mica, amphibole - and minor amounts of others.

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Minerals are made of elements

8 common elements compose 98% of continental crust rocks Some minerals (e.g., quartz) are composed of just two elements Others (e.g., amphibole) are made up of several elements Some elements occur more frequently than others The most common minerals in granite are quartz, feldspar, mica, and amphibole.

Table 7.1 Common Elements in Continental Crust

   

NA1.5NAOthers salt nuts, Bread,2.1MgMg Magnesium

fertilizer nuts, fruit, Fish,2.6KK Potassium cheese bacon, Salt,2.8NaNa Sodium

antacids cement, cheese, Milk,3.6CaCa Calcium caryour ,cornflakes Meat,5.0Fe ,FeFe Iron

aircraft Cans,8.1AlAl Aluminum chipscomputer glass, Window27.7SiSi Silicon

Air46.6OO Oxygen









in Found AlsoWeight by PercentIonElement

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Atom – smallest particle that retains the characteristics of an element

Atoms

Atoms are made up of protons, neutrons, and electrons Protons and neutrons in atomic nucleus Electrons in surrounding “cloud”

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Building Blocks of Minerals

Atomic number -The number of protons in the nucleus Each element has a different number of protons in the atomic nucleus Example: Neon has 10 protons, Helium has 2 protons Ions – atoms with different numbers of protons (positive) and electrons (negative) • Oxygen can gain two electrons to fill vacant sites • 8 protons, 10 electrons  -2 (negative charge, O2-) • Silicon may lose 4 electrons  +4 (positive charge, Si4+)

Elements bond together to form minerals

Ionic bonds – balance of negative and positive charges of different ions (e.g., rock salt) Covalent bonds – sharing of electrons between elements (e.g., diamond) to achieve a stable atomic structure

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Silicates

Multiple bonds – silicon and oxygen join together by a combination of ionic and covalent bonding 4 oxygen and one silicon atom combine by covalent bonds to form a silica tetrahedron (SiO4)

• Tetrahedron has a negative charge (4-) and forms ionic bonds with atoms of other elements

Silicate minerals – contain both silicon and oxygen Silicon and oxygen are most common elements in crust Silicates are the most common mineral group

• Examples: quartz, feldspar, mica, amphibole

Different types of bonds result in minerals of different strengths

Type of bonds determine strength of minerals, rocks • Ionic bonds – Velcro analogy, weaker bonds • Covalent bonds – Rope analogy, stronger bonds

Minerals formed with covalent bonds are stronger and more resistant to destructive forces at Earth’s surface • Silicates form more resistant rocks than most

other mineral groups

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Rocks and Minerals Conceptest

O2HCaSO D.

OFe C.

OKAlSi B.

FeS A.



Which of the following mineral formulae represents a silicate?

Five Characteristics of a Mineral

• A mineral is naturally occurring (formed naturally rather than manufactured).

• It is a solid.

• A mineral has a definite chemical composition, with slight variations.

• It is inorganic.

• It has a characteristic crystalline solid (a specific orderly, repeating arrangement of atoms, ions, or molecules).

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Crystal Form

Crystal form—crystal shape—is the outward expression of a mineral’s internal arrangement of atoms.

• Internal atomic arrangement is determined by atom (or ion) charge, size, and packing.

• The conditions in which the crystal grows also affect crystal form.

• Temperature, pressure, space for growth

• Well-formed minerals are rare in nature—most minerals grow in cramped, confined spaces.

• Common shapes are • Prisms • Pyramids • Needles • Cubes • Sheets

Hardness • Hardness is the resistance of a mineral

to scratching. • Hardness is dependent on the strength

of a mineral's chemical bonds. • The stronger the bonds, the harder the

mineral.

• Bond strength is determined by ionic charge, atom (or ion) size, and packing.

• Charge—the greater the attraction, the stronger the bond.

• Size and packing—small atoms pack more closely, resulting in a smaller distance between atoms, increasing the attractive forces and thus yielding a stronger bond.

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Color • Color is an obvious feature of many minerals, but it is not reliable for

mineral identification. • Very slight variations in composition or minor impurities can change a

mineral's color.

• Color results from the interaction of light waves with the mineral.

Color-Related Characteristics

• Streak is the color of a mineral in its powdered form. • Powder is produced by rubbing against an unglazed porcelain plate—a streak

plate. • Mineral color may vary, but streak color is generally constant.

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Color-Related Characteristics • Luster describes the way a mineral's

surface reflects light. There are two types of luster—metallic and nonmetallic.

Cleavage and Fracture

Cleavage is the property of a mineral to break along planes of weakness.

• Planes of weakness are determined by crystal structure and bond strength.

• Minerals break along planes where the bond strength is weakest.

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Cleavage – minerals break along planes of weakness defined by atomic structure

a. One direction b. two directions that intersect at 90° angles c. two directions that do not intersect at 90° angles d. three directions, intersecting at 90° angles e. three directions, not intersecting at 90° angles f. four directions (for example, diamond) g. six directions. Some minerals have no cleavage

planes (for example, quartz), while others may have several.

Cleavage and Fracture

Cleavage is the property of a mineral to break along planes of weakness.

• Fracture occurs in minerals where the bond strength is generally the same in all directions.

• Minerals that fracture do not exhibit cleavage.

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• Shows the interrelationships among the three rock types

• Magma • Crystallization

• Igneous rock • Weathering, transportation, and deposition

• Sediment • Lithification

• Sedimentary rock • Metamorphism

• Metamorphic rock • Melting

• Magma

Rock Cycle

• Full cycle does not always take place due to “shortcuts” or interruptions

• e.g., Sedimentary rock melts • e.g., Igneous rock is metamorphosed • e.g., Sedimentary rock is weathered • e.g., Metamorphic rock weathers

Rock Cycle

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Origin of Magma

Role of heat • The temperature increases within Earth's upper crust—the geothermal

gradient—at an average of 30°C per kilometer. • Rocks in the lower crust and upper mantle are near their melting points. • Any additional heat (from rocks descending into the mantle or rising heat

from the mantle) may help to induce melting. • Heat is a minor player.

Origin of Magma

Role of pressure • Reduced pressure lowers the melting temperature of rock. • When confining pressures drop, decompression melting occurs. • Analogies and examples:

• The solid inner core • A pressure cooker

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Origin of Magma

Role of fluids (volatiles) • Fluids (primarily water) cause rocks to melt at lower temperatures. • This is particularly important where oceanic lithosphere descends into the

mantle. • Analogies:

• Salt on icy roads • Antifreeze in a car's radiator

Summing Up: Three Factors of Magma Formation

• Temperature • Added heat can cause melting; this is a minor player.

• Pressure increases with depth • Convective motion in the mantle allows rock to rise upward, reducing the

pressure enough to lower the melting point and induce melting. • Addition of water to rock

• As rock is dragged downward during subduction, water-rich fluids are released and migrate upward.

• Fluids lower the melting point of overlying rock, allowing partial melting and magma generation.

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• Form as magma cools and crystallizes • Rocks formed inside Earth are called plutonic or intrusive

rocks • Rocks formed on the surface

• Formed from lava (a material similar to magma, but without gas • Called volcanic or extrusive rocks

• Crystallization of magma • Ions are arranged into orderly patterns • Crystal size is determined by the rate of cooling

• Slow rate forms large crystals • Fast rate forms microscopic crystals • Very fast rate forms glass

Igneous Rocks

• Classification is based on the rock’s texture and mineral constituents

• Texture • Size and arrangement of crystals

• Types of igneous textures • Fine-grained – fast rate of cooling • Coarse-grained – slow rate of cooling • Porphyritic (two crystal sizes) – two rates of cooling • Glassy – very fast rate of cooling • Vesicular – contains hole left by gas bubbles • Pyroclastic – fragmented; produced by consolidation of

volcanic fragments

Igneous Rocks

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Fine-Grained Igneous Texture

Coarse-Grained Igneous Texture

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Porphyritic Igneous Texture

Glassy Igneous Texture

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Vesicular Igneous Texture

Pyroclastic Igneous Texture

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• Naming igneous rocks • Granitic rocks

• Composed almost entirely of light-colored silicates – quartz and feldspar

• Also referred to as felsic: feldspar and silica (quartz) • High silica content (about 70 percent) • Common rock is granite

Igneous Rocks

Granite and Granitic Rock

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• Naming igneous rocks • Basaltic rocks

• Contain substantial dark silicate minerals and calcium-rich plagioclase feldspar

• Also referred to as mafic: magnesium and ferrum (iron) • Common rock is basalt

Igneous Rocks

Basaltic Lava

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• Naming igneous rocks • Other compositional groups

• Andesitic (or intermediate) • Ultramafic

Igneous Rocks

Harzburgite

Andesite

• Form from sediment (weathered products) • About 75 percent of all rock outcrops on the continents • Used to reconstruct much of Earth’s history

• Clues to past environments • Provide information about sediment transport • Rocks often contain fossils

• Economic importance • Coal • Petroleum and natural gas • Sources of iron and aluminum

Sedimentary Rocks

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• Classifying sedimentary rocks • Two groups based on the

source of the material • Detrital

• Material is solid particles • Classified by particle size • Common rocks include

• Shale (most abundant) • Sandstone • Conglomerate

Sedimentary Rocks

Common Clastic Sedimentary Rocks

• Shale • Composed of mud-sized particles in thin

layers • Most common sedimentary rock

• Sandstone • Composed of sand-sized particles • Quartz is the predominant mineral

• Conglomerates • Composed of particles greater than 2

mm in diameter • Consists largely of rounded gravels

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Formation of Sedimentary Rocks

• Weathering is the physical breakdown and chemical alteration of rock at or near Earth's surface.

• There are two types of weathering: • Mechanical weathering—breaking and disintegration of rocks into smaller

pieces • Chemical weathering—chemical decomposition and transformation of rock

into one or more new compounds

Formation of Sedimentary Rocks

• Erosion is the physical removal of material by mobile agents such as water, wind, ice, or gravity.

• Transportation: As sediment is transported, it continues to weather and erode. Particle size decreases and edges are rounded off.

• Deposition occurs when eroded sediment comes to rest.

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Formation of Sedimentary Rocks

• Sediment particles are deposited horizontally layer by layer. • As deposited sediment accumulates, it lithifies—changes into

sedimentary rock. • Lithification occurs in two steps:

• Compaction • Cementation

Formation of Sedimentary Rocks

• Compaction—Weight of the overlying material presses down on deeper layers.

• Sediment particles compact and squeeze together.

• Cementation—Compaction releases "pore water" rich in dissolved minerals.

• This mineralized "pore water" acts as a glue to cement sediment particles together.

• Calcite • Silica • Iron Oxide

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Classifying Sedimentary Rock • Chemical sedimentary rocks

consist of precipitated material that was once in solution.

• Precipitation of material occurs in two ways:

• Inorganic processes • Organic processes (biochemical

origin)

Chemical Sedimentary Rocks

Limestone • Limestone is the most abundant chemical rock. • It is composed chiefly of the mineral calcite. • Marine biochemical limestones form as coral

reefs, coquina (broken shells), and chalk (microscopic organisms).

• Inorganic types of limestone include travertine.

• Found in caves, caverns, and hot springs

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Chemical Sedimentary Rocks

Evaporites • Evaporation triggers the deposition of chemical precipitates. • Examples include rock salt and rock gypsum.

Chemical Sedimentary Rock

Coal: • Coal is different from other rocks because it is composed of organic

material. • Stages in coal formation (in order):

• Plant material • Peat • Lignite • Bituminous coal • Anthracite coal

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• Features of sedimentary rocks • Strata, or beds

(most characteristic) • Bedding planes separate strata • Fossils

• Traces or remains of prehistoric life • Are the most important inclusions • Help determine past environments • Used as time indicators • Used for matching rocks from different places

Sedimentary Rocks

• “Changed form” rocks • Produced from preexisting

• Igneous rocks • Sedimentary rocks • Other metamorphic rocks

• Metamorphism • Takes place where preexisting rock is subjected to

temperatures and pressures unlike those in which it formed

• Degrees of metamorphism • Exhibited by rock texture and mineralogy • Low-grade (e.g., shale becomes slate) • High-grade (obliteration of original features)

Metamorphic Rocks

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• Metamorphic agents • Heat • Pressure (stress)

• Confining pressure – from burial • Differential stress – occurs during

mountain building • Chemically active fluids

• Mainly water and other volatiles • Promote recrystallization by enhancing

ion migration

Metamorphic Rocks

• Metamorphic settings • Contact, or thermal, metamorphism

• Occurs near a body of magma • Changes are driven by a rise in temperature

Metamorphic Rocks

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Metamorphic Rocks

• Regional metamorphism • Directed pressures and

high temperatures during mountain building

• Produces the greatest volume of metamorphic rock

Confining Pressure and Differential

Stress

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Metamorphic Grade

• Metamorphic textures • Foliated texture

• Minerals are in a parallel alignment • Minerals are perpendicular to the compressional force

• Nonfoliated texture • Contain equidimensional crystals • Resembles a coarse-grained igneous rock

Metamorphic Rocks

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• Common metamorphic rocks • Foliated rocks

• Slate • Fine-grained • Splits easily

• Schist • Strongly foliated • “Platy” • Types based on composition (e.g.,

mica schist)

Metamorphic Rocks

Common Metamorphic

Rocks

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• Common metamorphic rocks • Foliated rocks

• Gneiss • Strong segregation of silicate minerals • “Banded” texture

• Nonfoliated rocks • Marble

• Parent rock is limestone • Large, interlocking calcite crystals • Used as a building stone • Variety of colors

• Quartzite • Parent rock – quartz sandstone • Quartz grains are fused

Metamorphic Rocks

• Metallic mineral resources • Gold, silver, copper, mercury, lead, etc. • Concentrations of desirable materials are

produced by • Igneous processes • Metamorphic processes

• Most important ore deposits are generated from hydrothermal (hot-water) solutions

• Hot • Contain metal-rich fluids • Associated with cooling magma bodies

Resources from Rocks and Minerals

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Evidence for the Impact Hypothesis in the Rock Record

• The impact hypothesis states that the Cretaceous extinction followed an asteroid impact.

• Evidence: • Iridium is a rare element on Earth but common in asteroids. • The position of an iridium layer in the rock record matches the time of the

Cretaceous extinction. • Shocked quartz • Impact crater at Chicxulub • Layer of soot in rock record dating to time of asteroid impact

One Inch Iridium - layer