Mineralogy

Mineralogy is a subdiscipline of geology which is devoted to the study of the nature, composition, and modes of occurance of those purest of rocks - Minerals. Minerals are naturally occuring substances which have characteristic internal structures comprised of regular arrangements of atoms and ions. Each mineral has a characteristic chemical composition and physical properties which vary within a definite range. For example, quartz is still quartz regardless of its color. Its internal structure is the same whether it is blue, red, or green, or any other color.

Minerals are used in many different human activities ranging from heavy industry to the finest jewelry. Gemstones are particularly perfect instances of naturally occuring minerals. All rocks are composed of specific combinations of minerals suites. Whether the rock is on the Moon, the Earth, or Mars it is composed of a mixture of commonly occuring minerals.

Geologists study minerals using many different tools ranging from simple hand lens inspection to x-ray diffraction and sophisticated chemical analyses. The chart below summarizes the characteristics of a selection of common minerals and presents the categories of description most frequently used. For a brief description of the categories used in this table, consult our glossary.

Mineral Chemical Formula Color Streak Hardness Specific Gravity Luster Cleavage Transparency Occurence Crystal System Uses Name Origin Picture(s)
Azurite Cu3(CO3)2(OH)2 azure-blue pale blue 3.5 - 4 3.77 vitreous one plane perfect; one plane fair transparent to translucent common monoclinic a valuable ornamental stone; also a minor ore of copper in allusion to its color Picture 1
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Calcite CaCO3 usually white to colorless, but may be tinted gray, red, green, blue, yellow, or brown or black when impure white 3 on cleavage; 2.5 on base 2.71 vitreous to earthy perfect rhombohedral transparent to translucent very common hexagonal mostly in the manufacture of cements and mortars; also as a soil conditioner; for whiting and whitewash; as flux in smelting; in lithography from the Latin work calx, meaning burnt lime Picture 1
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Copper Cu copper-red on fresh surfaces; usually tarnished dull brown or with a green film (malachite)   2.5 to 3 8.9 metallic none opaque very rare isometric mostly in electrical wire and alloys; many minor uses   Picture 1
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Fluorite CaF2 extremely variable; usually light green, yellow, bluish-green or purple; also colorless, white, rose, blue, brown white 4 3.18 vitreous perfect octahedral transparent to translucent very common isometric mainly in the preparation of hydrofluoric acid and as a flux in steelmaking from the Latin word fluere, to flow, because it melts more easily than other minerals with which it was confused Picture 1
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Graphite C black black 1 to 2 2.23 metallic; sometimes dull earthy perfect basal opaque rare hexagonal for refractories, electrodes, dry lubricants, pencils and dyes from the Greek word meaning to write, in allusion to its use in pencils Picture 1
Gypsum CaSO4 - 2H2O colorless, white, gray; various shades of yellow, red, brown from impurities white 2 2.32 usually vitreous; sometimes, pearly or silky perfect into thin flexible but inelastic plates transparent to translucent very common monoclinic in the making of Plaster of Paris and Portland cement; as a flux for pottery; as a fertilizer from the Greek name for the mineral Picture 1
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Magnetite Fe3O4 iron-black black 6 5.18 metallic some octahedral parting opaque very common isometric An important ore of iron   Picture 1
Malachite Cu2CO3-

(OH)2

 bright green bright green 3.5 to 4 3.9 to 4.03 adamantine to vitreous in crystals; silky in fibrous varieties; dull in earthy varieties perfect on one plane, but rarely seen translucent common monoclinic a valuable ornamental stone; also a minor ore of copper from the Greek word for mallows, in allusion to its green color Picture 1
Pyrite FeS2 pale brass yellow greenish or brownish black 6 to 6.5 5.02 metallic conchoidal opaque very common isometric A source of sulfur for sulfuric acid. Often mined for its association with gold or copper from the Greek word for fire. (It produces brilliant sparks when struck by steel.) Picture 1
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Quartz SiO2 usually colorless or white, but impurities may cause it to be any color white 7 2.65 vitreous; in some specimens, greasy, splendent none transparent to translucent very common hexagonal in pressure gauges, oscillators, resonators and wave stabilizers; in polarimeters; in heat-lamps, prisms and spectrographic lenses from a German word Picture 1
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Sulfur S yellow; impurities may cause color to be yellow shades of green, gray and red white 1.5 to 2.5 2.05 to 2.09 resinous none transparent to translucent rare orthorhombic primarily in the manufacture of sulfuric acid, hydrogen sulfide and other sulfur compounds; also in fungicides, fertilizers, explosives and in the vulcanization of rubber   Picture 1
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Tincalconite Na2B4O5-

(OH)4

-3H2O

 chalky white white 1 1.88 dull, earthy conchoidal opaque rare hexagonal a source of boric acid   Picture 1
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Tourmaline (Na,Ca)
(Li,Mg,Al)
(Al,Fe,Mn)
6-(BO3)3
(Si6O18)
(OH)4		 varied (depending on ions present) white 7 to 7.5 3.0 to 3.25 vitreous to resinous   transparent to translucent common hexagonal Some colored and transparent varieties are sold as semi-precious stones. Also used in high-pressure gauges (because of its piezoelectric property). from turamali, as gems from Sri Lanka were once called Picture 1
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Vanadinite Pb5(VO4)3

Cl

 ruby-red, orange-red, brown, yellow white 3 6.9 resinous to adamantine none transparent to translucent rare hexagonal an ore of vanadium and minor ore of lead; used in metal alloys and in dyes after its composition Picture 1
Wulfenite PbMoO4 after its composition white 3 6.8 vitreous to adamantine one good cleavage plane transparent to subtransparent rare tetragonal a secondary ore of molybdenum after Austrian mineralogist, X. F. Wulfen Picture 1
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