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Minerals IIElementary MineralogyGS 122Patterns in Nature: Minerals

Chemical bonding: Focus on covalent bonds

Mineral polymorphs

Physical properties of minerals

Common rock-forming silicate minerals

Ionic Bonding

Example: Table Salt: Sodium (Na) and Chlorine (Cl)Sodium gives up an electron becoming a positively-charged charged cation.

Chlorine picks up an electronbecoming a negatively charged anion.Atomic BondingBonding between sodium and chlorine in halite is based on these charge differences.

Sharing Electrons: Covalent BondingShared electronsNucleus4Factors that determine the internal structure of minerals:1) Composition of magma or fluids from which the minerals form.

2) Conditions under which the mineral forms: Temperature PressureMinerals comprised of the same elements in the same proportions can possess markedly different internal structures.

For example:Higher pressure -> Denser packing of atoms -> Different mineralMineral Structure & Conditions of FormationDifferent minerals w/ same chemical composition ,but differing structures, are called polymorphs Graphite (a form of pure carbon) Soft gray material, e.g., pencil lead Crystal structure: sheets of carbon

Diamond (also pure carbon) Forms deep in Earth at high pressures, & is hardest substance known to humans Crystal structure: dense & compact

Color Luster Hardness Streak Crystal form Cleavage FractureTo identify minerals, we use their physical and optical properties. Some properties are more diagnostic than others, so we try to use a combination when making a determination. Useful properties include: Reaction to acid Taste Smell Magnetization Optical properties Elasticity Specific gravityIdentifying Minerals7 ColorObvious, but often misleading. Slight impurities in a mineralcan change its color.

Example: Quartz (when pure it is colorless), but there are many color varieties which result from small amounts of other elements.

Physical properties of minerals8 LusterThe appearance of light reflected from minerals.

Examples:

Metallic luster vs. nonmetallic luster

Glassy (vitreous) luster

Resinous lusterPhysical properties of minerals9 HardnessVery useful! Measures a minerals resistance to scratching. We use Mohs hardness scale (below) for comparisons.

Physical properties of minerals10

Crystal Form Reflects the Internal Arrangement of Atoms

Crystal form in halite (salt; NaCl) is cubic

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A law of mineralogy: Constancy of angles between crystal faces13 Crystal FormThe shape of a well-formed crystal reflects directly the orderly internal arrangement of its constituent atoms. Well-formed crystals that grow without interference are called euhedral.

Quartz(SiO2)

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Euhedral crystals of quartz

Quartz geodeAnhedral crystals formed by crowding during growth16

Anhedral quartz crystals formed by crowding during growth

Crystal terminations of euhedral quartz17

Euhedral crystals of amphibole in a volcanic rock18

CLEAVAGE Tendency to break along preferred planes of weakness. Cleavages represent directions of weaker bonding between atoms.19

2-directional cleavage in mica20

In mica, atoms are arranged in weakly-connected sheets21

AsbestosCleaves into long flexible fibers22

group of silicate minerals that readily separate into fibers that are: thin, flexible, heat resistant, chemically inert

=> many uses

- mainly three types:

chrysotile (white asbestos)crocidolite (blue asbestos)amosite (brown asbestos)AsbestosAsbestos23

Cleavage in both Halite (salt) & calcite (lime) is in three directions. But the angles between cleavages are different for these minerals.Halite has a cubic cleavage.Calcite cleaves into rhombohedra.

3-directional cleavage24 Cleavage:The tendency of a mineral to break along planes of weak bonding in the crystal structure. The number and angles between cleavgae faces are very useful properties for identification.

Mica (sheets)Calcite(rhombs)

Halite(cubes)In Summary.25

Bond strengths are equal in all directions. No preferred directions of weakness. Quartz does not cleave, but breaks along smooth, curved, glassy surfaces. Called conchoidal (glassy) fractureConchoidal Fracture in Quartz26

Conchoidal fracture in volcanic glass27

Streak: Color of mineral in its powdered formHematite: Iron oxide28

Carbonate minerals, like calcite, dissolve in acid and release carbon dioxideThe Acid TestCO2 bubbles29HalidesHalite (Na, Cl: NaCl)-> common table saltSulfatesGypsum (Ca,S,O,H: CaSO4-H2O)-> calcium sulfate + water, main ingredient of plaster & other building materialsOxidesHematite (Fe, O: Fe2O3)-> steelImportant Non-silicate Minerals30Carbonates

Calcite (Ca, C, O: CaCO3)Dolomite (Ca, Mg,C, O: CaMg(CO3)2 Found together in sedimentary rock limestone.

Main ingredient to cement, roads & building stones.Important Non-silicate Minerals31Over 4000 minerals: only few dozen are abundant, making up most rocks of Earths crust => rock-forming minerals

Only 8 elements make up most of crusts minerals & represent over 98% of the continental crust

The two most abundant elements:

Silicon (Si) Oxygen (O)The Common Rock-forming Minerals32

Average composition of the Earths crust.Percent of elements by WEIGHTQuestion: What minerals would you expect to be most abundant on Earth?33Earths CrustPrimarily Si & O followed in abundance by Fe, Mg, Ca, Na, K, etc.

Dark colored (mantle and oceanic crust)Olivine (Si, O, Fe, Mg)Pyroxene (Si, O, Fe, Mg, Ca)Amphibole (Si, O, Fe, Mg) Light colored (crust, esp. continental crust)Quartz (SiO2) - Hard, transparentFeldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink Clay (Mostly come from weathering feldspar) Calcite (CaCO3, shells) Limestone - Used for cement

The Common Rock-forming Minerals34

Si4+O2-O2-O2-O2-SiO44-Silicon tetrahedron hasAn overall charge of -4

1 silicon (Si) atom4 oxygen (O) atomsBasic Building Block of Silicate Minerals:The Silicon-Oxygen TetrahedronAn anion with charge of -435Tetrahedra link up by forming covalent bonds between oxygen atoms:Single silicon tetrahedron:A silicon atom covalently-bonded to four oxygens.Two tetrahedra can joinby sharing an electronbetween adjacent oxygenatomsSilicates: The Common Rock-forming MineralsBasic Building Block: The Silicon-Oxygen TetrahedronOxygen atomSilicon atom36

37SilicatesThe Common rock-forming minerals

Silicon-oxygen tetrahedra can be arranged into:Single chains: PyroxeneDouble chains: AmphiboleSheets: Micas38 Silicate chains and sheetsNot electrically neutral!Unsatisfied negative charges of oxygenslocated at the edges of chains, or between sheets are neutralized by coordinating metallic ions at those sites.

Balancing Charges in Silicates: Role of Metal CationsIron (Fe)Magnesium (Mg)Potassium (K)Sodium (Na)Aluminum (Al)Calcium (Ca)

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Ionic SubstitutionIons of similar size (ionic radius) and charge can substitute for one another in a mineral.

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