Minerals are: 1) naturally occuring 2) inorganic 3) solids with 4) definite chemical compositions and 5) an orderly arrangement of atoms.

There are over 2000 different minerals. A few common "rock-forming minerals" make up most of the rocks in earth's crust, and most of these (over 90 percent) are silicates. The minerals are grouped based on their chemical composition. A few native minerals (gold, silver, copper, sulfur, diamond, ...) are made of a single element. Most minerals, however, are formed of compounds -- two or more chemically combined elements. Listed below are some important groups of rock-forming minerals. For a more extensive list of minerals and mineral groups you can use a rock & mineral guide or visit other online resources.

*Know the bold terms & minerals for chapter test

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Common Rock-Forming Minerals

Silicates: minerals made of silica tetrahedrons (silicon + oxygen), alone or combined with other elements

Silicate Families :

• quartz - made completely of silica tetrahedrons; hardest common mineral (H=7 on the Moh's scale) and second most abundant in the earth's crust; color varies, but has a glassy luster ; found in granites, sand, and sandstone
• feldspar - most abundant family of minerals in earth’s crust, includes:
  • orthoclase (most common example of potassium feldspar) - cleaves in two directions that meet at 90 degrees; light color with a pearly luster; H=6; common in granites
  • plagioclase (sodium-calcite) feldspars - cleaves in two directions that meet at oblique angles; one surface often has striations (fine parallel lines) ; color varies from white to reddish brown; H=6; examples include albite & oligoclase
• mica - soft (H=2.5), flaky mineral; non-metallic luster; excellent cleavage in one direction that peels into thin elastic layers; includes muscovite (silvery white) and biotite (dark - brown/black)
• talc - softest mineral (H=1); color is white, gray or geenish; good cleavage in one direction; soapy feel; used in powders
• hornblende - most common mineral in the amphibole family of silicates; dark color; shiny luster; H=5-6; 2 good cleavages meet at oblique angles; form long slender crystals; common in many dark crystalline rocks
• augite - most common pyroxene mineral; dark color; H=5-6; two good cleavages at nearly 90 degrees; less luster than horneblende; forms stubby crystals; common in many dark crystalline rocks
• olivine - olive (yellowish) green; glassy luster; forms granular (grain-like) crystals; hard (about 6.5); found in dark crystalline rocks and some meteorites
• garnet - very hard (6.5 - 7.5); found in many crystalline rocks
• kaolin - aluminum silicate; forms from weathering of feldspars & other silicates; primary mineral in clay & shale

ferromagnesian silicates: dark colored minerals containing iron & magnesium; may belong to almost any of the silicate families; examples include hornblende, augite, olivine

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Carbonates: made of carbonate group (carbon + oxygen) joined with various metal ions; found in limestone and marble

• calcite - "calcium carbonate"; clear or white when pure - may be any color however due to impurities; H=3; three perfect cleavages are oblique; forms rhombs; reacts to readily to acid; most common carbonate mineral
• dolomite - calcium magnesium carbonate; doesn’t react as easily to acid as calcite; found in some limestones & marbles
• copper carbonates include malachite (blue) and azurite (green); an iron carbonate is siderite (brown or yellow-brown)

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(Iron) Oxides: oxides are compounds in which oxygen combines with metal elements (such as iron)

• hematite - most common iron oxide; H=5-6; earthy red or silvery metallic luster; streak is always red-brown
• magnetite - dark iron oxide; H=5.5-6.5; attracted to a magnet; forms small grains or crystals in many rocks
• lodestone - magnetic form of magnetite; "natural magnet"

Other oxides:

• corundum, ruby & sapphire - aluminum oxides (Al2O3); H=9; SG=4.0-4.1; conchoidal to uneven fracture; used in gems

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(Iron) Sulfides: sulfides are compounds in which sulfur combines with metal elements (such as iron)

• pyrite - most common sulfide mineral; pale brass to golden-yellow color; metallic luster; H=6-6.5; may form 6- or 12-sided crystals; common name is “fool’s gold”
• chalcopyrite - iron sulfide which also contains copper (Cu); brassy yellow, tarnishes to iridescent blues, greens & purples (sometimes called "peacock copper"); metallic H=3.5-4; contains less than a third copper, but because it is found in large quantities, it is the major source of copper

Other sulfides:

• galena - ore containing 86% lead; excellent three directions of cleavage at 90 degrees; bluish lead gray color; metallic luster (when freshly broken); very dense; it is also the leading ore for silver, even though it only contains small amounts
• cinnabar (HgS) - an ore of mercury; H = 2-2.5; color is red; streak is scarlet

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Other Minerals

Halides: compounds in which metallic element combines with halogens (F, Cl, Br, I)

• halite - rock salt (NaCl); cleaves into cubes; can be identified by taste (not recommended)
• flourite (CaF2) - Color varies; H = 4; colorless streak; nonmetallic luster; cubic or octahedral crystals; cleaves in four directions (forming triangular shapes); some samples exhibit flourescense (glow under UV light)

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Sulfate: compounds in which metal elements combine with sulfate radical (SO4)-2

• gypsum - clear, white, yellow or reddish; vitreous luster; very good one direction cleavage; soft (H=2); forms of gyspsum used as base in plasters & cements, as well as fertilizers and filler for paper

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Native Minerals: minerals composed of single (uncombined) elements

• sulfur (S) - yellow; H=1.5-2.5; indistinct cleavage; earthy luster; may smell like rotten egg; burns easily (and has low melting point), but gives off a poisonous gas
• diamond (C) - native mineral made of carbon; H=10 (hardest) on the Moh's scale
• copper (CU) - H = 2.5 - 3; copper red color; metallic luster ; S.G. = 8.5 - 9
• gold (Au) - H = 2.5 - 3; gold color; metallic luster; dense

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Identification of Minerals

class set of minerals	mineral tests: acid, magnetism, hardness	measuring specific gravity

Minerals are identified by their physical & chemical properties.

Inspection: examination of samples using sight (or touch/feel)

• color - most obvious, but often least useful property because a) many minerals have the same color, b) the same mineral can have different colors due to "impurities" (trace amounts of other elements), and 3) some minerals change color when exposed to air
• luster - how light reflects off mineral sufaces, includes: metallic and nonmetallic (vitreous/glassy, pearly, dull, earthy...)
• crystal shape - intersection of the crystal faces; many crystals lacked time or room to develop adequately for visual inspection

Simple Tests:

• streak - color of minerals powder left on a streak plate
• cleavage - tendency to split along flat surfaces; number of cleaveage planes (one, two, three, or none) and angle of intersection between the planes (90 degrees or oblique) can help in identification of some mineral
• fracture - minerals that don't cleave; includes conchoidal (shell-like), splintery, irregular, ...
• hardness - measured using Moh's scale; 1 (talc - softest mineral) to 10 (diamond- hardest mineral)

Other Tests:

• specific gravity - ratio of the weight of a mineral to the weight of an equal volume of water (compares density of mineral to that of water)

specific gravity = (weight of mineral in air) / (weight in air minus weight in water)

Example: (measurements of a mineral tested in class)

mass of mineral in air = 30 grams
mass of mineral in water = 18.5 grams
difference in mass (equals the mass of an equal volume of water) = 30.2 grams - 19.0 grams = 11.2 grams

specific gravity = 30.2 grams / 11.2 grams = 2.70

This measurement helped confirm the mineral's identity as calcite. According to the Dorling Kindersley handbook, Rocks and Minerals by Chris Pellant, the specific gravity of calcite is 2.71.

• acid test - calcite found in limestone and marble reacts readily to acid by fizzing (produces carbon dioxide gas)

Special Properties:

• magnetism - attracted to magnet (magnetite) or even acts as a magnet (lodestone)
• taste - halite (rock salt) can be identified using taste, but this is NOT a recommended test
• fluorescence - glows under ultraviolet light (flourite, ...)
• radioactivity - subatomic particles released are measured with a Geiger counter
• double refraction - splits light rays into two parts (Island spar -- a transparent variety of calcite)

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