Rocks and Minerals: A First Look

Chapter 2

The differences in the physical properties of

rocks, minerals, and soils determine 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

Atoms• Smallest particle into which an element can be

divided while still retaining the chemical characteristics of that element

• Composed of a nucleus surrounded by electrons

– Nucleus is composed of protons (+) and neutrons (0)– Number of protons defines the chemical element and

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

Figure 2.1 Schematic drawing of atomic structure

Elements and Isotopes• Element – substance composed of atoms with the same

number of protons• All nuclei, except the simplest hydrogen atoms, contain

neutrons• The number of neutrons is similar to or somewhat

greater than the number of protons• Isotopes – number of neutrons for an element may not

be the same; variable numbers of neutrons possible– Atomic Mass Number is the number of protons and neutrons in

the element’s 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 6 neutrons)

Ion• An atom that is positively charged or

negatively charged– Anion has gained electrons (-); has more electrons

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

relative to the number of protons (+)

• The electrical attraction of ions will cause an ionic bond to form between oppositely charged ions.

– Na+ + Cl- = NaCl (halite)

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

MINERALS

• Naturally occurring

• Inorganic

• Solid element or compound

• Definite chemical composition

• Regular internal crystal structure

• Identified by recognizing different physical properties

Figure 2.3A

Figure 2.3B

Identifying Minerals

• The two fundamental characteristics of a mineral 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

Mineral Physical Properties

• Color• Hardness• Cleavage• Luster• Density• Crystalline Form

Mineral CompositionSilicate Group

• Silicate group – variety of compounds based on silicon and oxygenQuartz – glass manufacturingFeldspar – ceramic manufacturingMica

• Muscovite (white mica)• Biotite (black mica)

Clays – used as drilling mud, in building materials, and as a soil modifier

• Ferromagnesian silicatesOlivine – peridot (semiprecious gem)Garnet – abrasives; semiprecious gemsAmphibole – industrial products

Figures 2.7 a and b

Mineral CompositionNonsilicates

• Carbonates – CO3

– Useful for building materials and manufacturing

• Sulfates – SO4

– Useful for building materials

• Sulfides – S– Host for many metallic ores (Pb, Cu, Zn, and others)

• Oxides – any metal combined with oxygen– Iron and aluminum ores

• Native elements – minerals composed of single element– Carbon as diamond and graphite– Copper, gold, silver, or platinum

Rocks – formed 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

Igneous Rocks• Magma, at high enough temperatures, rocks

and minerals melt, and the natural hot, molten rock material is called magma

• Silicates are the most common minerals, and magmas are thus rich in silica.

• Magmas also contain some dissolved water and gases, and include some solid crystals suspended in the melt; iron content is variable and this variability is the basis for igneous rock classification

• An igneous rock is a rock formed by the solidification and crystallization of a

cooling magma

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

– Plutonic rocks form if magma cools inside earth’s crust (does not flow onto surface); coarse crystals will grow

– Volcanic rocks form if magma flows onto surface as lava; glass often forms

Igneous Rocks*Know this chart

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

Weathering of Rocks

• Chemical weathering

• Physical weathering• Creates Sediments• Ions for cements

Sedimentary Rocks• Sediments are:

produced by weathering of

pre-existing rocks and minerals

loose, unconsolidated accumulations of mineral or rock particles

eroded, transported, and deposited in many sedimentary environments

buried and experience lithification

Lithification involves compacting the sediments with burial and cementation of the sediments forming a sedimentary rock

Sedimentary Rock FACTS:• 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.

• They yield information about the settings or environment in which the sediments were deposited.

• They are formed at or near the earth’s surface and at temperatures close to ordinary surface temperatures.

2 Types of Sedimentary Rocks

1.Clastic sedimentary rocks– Formed by the lithification of mechanically weathered

pieces of rocks and minerals (fragments)• Grain sizes range from boulder, gravel, sand, silt, and mud

– Once deposited these clastic particles a cementedie. Conglomerate, sandstone…

2.Chemical sedimentary rocks– Chemical process occur in water bodies such as lakes,

seas, or oceans (ie. evaporation)

– Minerals precipitate from the water and form thick deposits• Examples: Halite, Calcite, and Gypsum

Figures 2.11 Sedimentary Rocks

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

Metamorphic Rock Changes• 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• Sources of elevated temperatures of metamorphism:

burial, magma, mountain-building, and plate tectonic movement

• Sources of elevated pressures of metamorphism: burial, mountain-building,

and plate tectonic movement

Types of Metamorphism

• Contact metamorphism – localized metamorphism of rocks adjacent to

a magma chamber

• Regional metamorphism – large scale stressing and heating of a rock by deep

burial or continental plates moving and colliding

Common Metamorphic RocksAny kind of pre-existing rock can be metamorphosed

• Foliation: when a rock is subjected to directed stress, its minerals form elongated/platy crystals and line up parallel to each other

1. Nonfoliated Rocks do not show directed stress– Marble is metamorphosed limestone– Quartzite is metamorphosed quartz-rich sandstone

2. Foliated Rocks show directed stress or pressure– Slate is low grade foliated metamorphic rock– Schist and Gneiss is high grade metamorphic rocks

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

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

The Rock Cycle

Rocks grouped on the basis of their mode of origin:

igneous, sedimentary, and metamorphic

Shows the interrelationships between the three rock types

Rocks of any type can be transformed into rocks of another type or into another distinct rock of the

same general type through the geologic processes

Rocks are continually being changed by geological processes

The Rock Cycle • Earth as a system: the Full cycle

• Magma, a molten material formed inside Earth• Crystallization, magma cools and solidifies• Igneous rock: formed by “fire” underneath• Weathering, transportation, and deposition

(sediment formation )• Lithification • Sedimentary rock

• Metamorphism (heat and pressure)

• Metamorphic rock • Melting

• Magma Forms

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

Fig. 2.13 The Rock Cycle