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Rocks and Minerals: A First Look

Chapter 2

The differences in the

physical properties of

rocks, minerals, and soilsdetermine their suitability

for different purposes

extraction of water or of

metals, construction,manufacturing, waste

disposal, agriculture, and

other uses

Illinois Fluorite, photo by J. Carr

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Atoms Smallest particle into which an element can be

divided while still retaining the chemicalcharacteristics of that element

Composed of a nucleussurrounded byelectrons Nucleus is composed of protons(+) and neutrons

(0)

Number of protons defines the chemical elementand atomic number( H = 1, He = 2, Li = 3, )

Number of neutron adds mass to the atom Number of electrons (-) orbiting nucleus determined

by the number of positively charged protons;

Negatively charged electrons balance the positive

charges of the protons

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Figure 2.1 Schematic drawing of atomic structure

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Elements and Isotopes

Elementsubstance composed of atoms with thesame number of protons

All nuclei, except the simplest hydrogen atoms,contain neutrons

The number of neutrons is similar to or somewhatgreater than the number of protons

Isotopesnumber of neutrons for an element may not be thesame; variable numbers of neutrons possible

Atomic Mass Numberis the number of protons and

neutrons in the elements nucleus Some isotopes have more neutrons and are heavier

(carbon-14 has 6 protons and 8 neutrons)

Some isotopes have fewer neutrons and are lighter (carbon-12 has 6 protons and 6neutrons)

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Ion

An atom that is positively charged or

negatively charged

Anionhas gained electrons (-); has more

electrons relative to the number of protons (+) Cationhas lost electrons (-); has fewer electrons

relative to the number of protons (+)

The electrical attraction of ions will cause an

ionic bondto form between oppositelycharged ions.

Na++ Cl-= NaCl (halite)

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Compounds

Mixing of two or more chemical elements

in particular proportions that have

distinctive physical properties

Elements will bond because of electrical

attraction, forming ionic bonds, or the

atoms may share electrons, forming

covalent bonds

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MINERALS

Naturally occurring

Inorganic

Solid element or compound Definite chemical composition

Regular internal crystal structure

Identified by recognizing different physicalproperties

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Figure 2.3B

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Identifying Minerals

The two fundamental characteristics of amineral are its chemical composition and

its crystal structure

Analyze the mineral composition Technology based

Measure crystal structure and symmetry

Technology based Observe and measure physical and

special properties

Easy for humans to see and recognize

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Mineral Physical Properties

Color

Hardness

Cleavage

Luster

Density

Crystalline Form

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Figure 2.6

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Figures 2.7 a and b

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Mineral CompositionNonsilicates

CarbonatesCO3 Useful for building materials and manufacturing

SulfatesSO4 Useful for building materials

SulfidesS Host for many metallic ores (Pb, Cu, Zn, and others)

Oxidesany metal combined with oxygen Iron and aluminum ores

Native elementsminerals composed of singleelement Carbon as diamond and graphite

Copper, gold, silver, or platinum

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Figures 2.2 c and d

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Figures 2.3 a and b

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Figure 2.4

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Figures 2.5 a and b

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Rocksformed from Minerals

A solid aggregate of one or more

minerals, or mineral materials

Consists of many mineral grains or

crystals forming a solid mass

Each rock contains a record of its own

history

Three broad categories

Igneous

Sedimentary

Metamorphic

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Igneous Rocks

Magma, at high enough temperatures,

rocks and minerals melt, and the naturalhot, molten rock material is called magma

Silicates are the most common minerals,

and magmas are thus rich in silica.Magmas also contain some dissolvedwater and gases, and include some solidcrystals suspended in the melt; iron

content is variable and this variability is thebasis for igneous rock classification

An igneous rock is a rock formed by thesolidification and crystallization of acoolin ma ma

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Igneous Rocks

Rocks formed from hot, molten rock

material

Usually composed of silicate minerals and

some dissolved gases and water

Molten materials are very hot

Plutonicrocks form if magmacools inside

earths crust (does not flow onto surface);

coarse crystals will grow Volcanicrocks form if magma flows onto

surface as lava; glass often forms

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Figures 2.10 a and b

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Igneous Rocks

Felsic;

low iron

light toned

Intermediate Mafic;

high iron

dark toned

Ultramafic

Volcanic;extrusive;

aphanitic

Rhyolite Andesite Basalt Eclogite

Plutonic;

intrusive;

phaneritic

Granite Diorite Gabbro Peridotite

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Weathering of Rocks

Chemical weathering

Physical weathering

Sediments Ions for cements

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Sedimentary Rocks

Sediments are produced by weathering of pre-

existing rocks and minerals

Sediments are loose, unconsolidated

accumulations of mineral or rock particles

Sediments are eroded, transported, and depositedin many sedimentary environments

The sediments will be buried and experience

lithification

Lithificationinvolves compacting the sediments

with burial and cementation of the sediments

forming a sedimentary rock

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Sedimentary Rocks

Gravity plays a role in the formation of all

sedimentary rocks.

Layering is a very common feature of

sedimentary rocks and is used to identify the

origins of sedimentary rocks.

Sedimentary rocks can yield information

about the settings in which the sediments

were deposited. Sedimentary rocks are formed at or near the

earths surface and at temperatures close to

ordinary surface temperatures.

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Types of Sedimentary Rocks

Clastic sedimentary rocks Formed by the lithification of mechanically weathered

pieces of rocks and minerals Grain sizes range from boulder, gravel, sand, silt, and mud

Grains are continually broken down in size and shape

until deposited Once deposited these clastic particles a cemented

Chemical sedimentary rocks Chemical process occur in water bodies such as

lakes, seas, or oceans Minerals precipitate from the water and form thick

deposits Examples: Halite, Calcite, and Gypsum

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Figures 2.11 Sedimentary Rocks

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Metamorphic Rocks

Changed form rock

Rock formed from pre-existing rock or minerals

Heat, pressure, and chemically active fluids

cause changes in rock

Heat increases as a rock is buried or is close to

a magma chamber

Pressure increases with burial or collision

between moving continents Fluids become heated and circulate with burial

or with location near a magma chamber

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Metamorphic Rocks

The temperatures required to form metamorphic

rocks are below magmatic temperatures Significant changes can occur in a rock at

temperatures well below melting

Temperature and pressure can cause the

minerals in the rock to recrystallize Pressure may cause the rock to be deformed

The sources of elevated temperatures ofmetamorphism: burial, magma, mountain-building, and plate tectonic movement

The sources of elevated pressures ofmetamorphism: burial, mountain-building, andplate tectonic movement

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Types of Metamorphism

Contact metamorphismlocalized

metamorphism of rocks adjacent to a

magma chamber

Regional metamorphismlarge scale

stressing and heating of a rock by deep

burial or continental plates moving and

colliding

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Common Metamorphic Rocks Any kind of preexisting rock (another rock) can

be metamorphosed Foliation:when a rock is subjected to directedstress, its minerals form elongated/platy crystalsand line up parallel to each other

Metamorphic rocks without foliationdo not showdirected stress Marble is metamorphosed limestone

Quartzite is metamorphosed quartz-rich sandstone

Metamorphic rocks with foliationshow directedstress or pressure Slatelow grade foliated metamorphic rock

Schist and Gneiss (nice)high grade metamorphic

rocks

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Figures 2.12Metamorphic rocks have undergonemineralogical, chemical, and/or structural change

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Figures 2.12 Metamorphic rocks have undergone

mineralogical, chemical, and/or structural change

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The Rock Cycle

Three rock groups on the basis of their modeof origin: igneous, sedimentary, andmetamorphic

Shows the interrelationships among the

three rock types Rocks of any type can be transformed into

rocks of another type or into another distinctrock of the same general type through thegeologic processes

Rocks are continually being changed bygeological processes

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The Rock Cycle

Earth as a system: the 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

Through time, geologic processes acting

on older rocks change them into new

and different ones so that, in a sense, all

kinds of rocks are interrelated

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Fig. 2.13 The Rock Cycle

Th R k C l

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The Rock Cycle

The Essence of the Rock Cycle

Igneous rocks, formed from magma Sedimentary rocks, formed from low-

temperature accumulations of particles or

by precipitation from solution Metamorphic rocks, formed from

preexisting rocks through the change oftemperature and pressure

Geologic processes working on old rockschange the old rocks into new anddifferent ones, and thus all kinds of rocks