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CHE 106: Geochemistry

Minerals

INTRODUCTION AND PURPOSE A mineral is a solid, homogenous, inorganic substance (compound or element) found occurring naturally in Earth's crust. Minerals possess a fairly definite chemical composition and a distinctive set of physical properties, which include hardness, color, streak, luster, cleavage, crystalline structure, specific gravity (density), and magnetism. Hardness refers to the ability of one mineral to scratch another. Hardness is easily identified by mutual scratching of the mineral with a common article which has been ranked by its hardness. The scale given below is used as a basis for comparing the hardness of some common minerals. Softest 1. 2. 2.2 3. 3.1 4. 5. 6. 7. 8. 9. 10. Talc, graphite Gypsum (or selenite, a variety of gypsum) Fingernail Calcite Copper Fluorite Apatite or glass Feldspar (or microcline, a variety of feldspar) Quartz Topaz Corundum Diamond (not generally included in hardness kits)

Hardest

Cleavage refers to the tendency of some minerals to break along definite smooth planes. Cleavage may occur in one to six directions. The mineral may exhibit distinct cleavage along one or more planes, or it may exhibit indistinct cleavage or no cleavage. The degree of cleavage that a mineral exhibits is a clue to the identification of the mineral. Note: record cleavage only if it is apparent in the mineral specimen; do not hit any specimen! Although the color of a mineral may be impressive, it is not a reliable property for identifying the mineral, since the presence of small amounts of impurities may cause drastic changes in the color of some minerals. Streak refers to the color of the powder of the mineral. A mineral may exhibit an appearance of several colors but it will always show the same streak. A mineral rubbed (streaked) across the surface of an unglazed porcelain tile will thereby be powdered and will show its true color. This will work only if the hardess of the mineral is less than that of the streak plate!

2 Luster refers to the appearance of the mineral's surface in reflected light. Mineral surfaces appear to have either a metallic or nonmetallic luster. A metallic luster has the appearance of polished metal; a nonmetallic appearance may be of varying lusters and the lusters likened to the materials as oppositely listed below: Adamantine Greasy Pearly Resinous Silky Vitreous appearance of a appearance of appearance of a appearance of appearance of appearance of Diamond Oily glass Pearl Yellow resins Silk Glass

Crystalline structure refers to the way the atoms or molecules that make up the mineral are arranged internally. This arrangement is a function of the size and shape of the molecules and the forces that bind them. Magnetism refers to the property of possessing a magnetic force field. A mineral possessing magnetism can be detected by a magnetic compass. After completing this experiment, you should be able to define a mineral, understand the methods used for identifying minerals, and use these methods to actually identify some minerals. EQUIPMENT Minerals specimens, hardness kits, magnets, streak plates, a magnifying glass, and a handbook on rocks and minerals. PROCEDURE Perform the necessary tests as described in the introduction on the assigned set of minerals. Record your observations on the Report Sheet, and identify each assigned mineral using the attached descriptions.

3 KEY TO MINERALS 1. Minerals harder than steel or glass 1.1 Minerals with distinct cleavage or with distinct crystal form 1.11 Minerals with colored streak 1.111 Pyrite 1.12 Minerals with uncolored, white, or pale streak 1.121 Hornblende 1.122 Potash (or Potassium) Feldspars (Variety: Orthoclase, Microcline) 1.123 Plagioclase Feldspar (Variety: Albite, Labradorite) 1.124 Rock Crystal Quartz 1.2 Minerals with indistinct cleavage or with no cleavage 1.21 Minerals with colored streak 1.211 Hematite 1.212 Magnetite 1.22 Minerals with uncolored, white, or pale streak 1.221 Chert 1.222 Flint 1.223 Milky Quartz 1.224 Rose Quartz 1.225 Jasper 1.226 Olivine 2. Minerals softer than steel or glass 2.1 Minerals with distinct cleavage or with distinct crystal form 2.11 Minerals with colored streak 2.111 Galena 2.112 Graphite 2.12 Minerals with uncolored, white, or pale streak 2.121 Biotite 2.122 Calcite 2.123 Gypsum (Variety: Selenite, Satin Spar, Alabaster) 2.124 Halite 2.125 Muscovite 2.126 Azurite 2.127 Sphalerite 2.128 Fluorite 2.129 Talc 2.2 Minerals with indistinct cleavage or with no cleavage 2.21 Minerals with uncolored, white, or pale streak 2.211 Malachite 2.212 Bauxite

4 MINERAL DESCRIPTIONS 1.111 PYRITE Hardness: 6 - 6.5 Streak: black to greenish Luster: metallic Color: brass yellow Chemical composition: FeS2 Specific gravity: 5.02 Comments: Often cubic in shape with striations on faces; also comes massive; conchoidal fracture; called "fool's gold" 1.121 HORNBLENDE Hardness: 5-6 Streak: green-gray Luster: vitreous Color: greenish black Chemical composition: Ca, Mg, Fe, Al silicate Specific gravity: 3.2 Comments: Long, columnar crystals; visible cleavage in 2 directions; widespread occurrence in igneous and metamorphic rocks, particularly the latter 1.122 POTASH (or POTASSIUM) FELDSPARS Variety: ORTHOCLASE Hardness: 6.2 Streak: white Luster: vitreous Color: white to gray Chemical composition: K(AlSi3O8) Specific gravity: 2.57 Comments: Common rock-forming mineral; monoclinic; two prominent cleavages making angle of 90° with each other Variety: MICROCLINE Hardness: 6 Streak: uncolored, white, or pale Luster: vitreous Color: buff, green, pink, gray Chemical composition: K(AlSi3O8) Specific gravity: 2.54 - 2.57 Comments: Well-developed cleavage in two directions

5 1.123 PLAGIOCLASE FELDSPARS Variety: ALBITE Hardness: Streak: Luster: Color: Chemical composition: Specific gravity: Comments: Two-directional cleavage

6 uncolored, white, or pale vitreous white or light-colored Na(AlSi3O8) 2.62

Variety: LABRADORITE Hardness: 6 Streak: uncolored, white, or pale Luster: vitreous Color: dark color Chemical composition: Ca(AlSi3O8) Specific gravity: 2.69 Comments: Bluish sheen produced by striations; occasionally shows iridescence 1.124 ROCK-CRYSTAL QUARTZ Hardness: 7 Streak: uncolored, white, or pale Luster: vitreous Color: colorless Chemical composition: SiO2 Specific gravity: 2.65 Comments: Six-sided crystals with striations on crystal faces; conchoidal fracture 1.211 HEMATITE Hardness: 5.5 - 6.5 Streak: red-brown, except when in powder, then streak is Indian red Luster: metallic (when in crystals) Color: black, dark brown, or red Chemical composition: Fe2O3 Specific gravity: 5.26 Comments: Has a metallic luster when in crystals; also comes in granular, fibrous, or massive forms-

6 1.212 MAGNETITE Hardness: 6 Streak: black Luster: metallic Color: black Chemical composition: Fe3O4 Specific gravity: 5.18 Comments: The variety of magnetite called Lodestone is a natural magnet and will strongly deflect a compass needle 1.221 CHERT Hardness: 7 Streak: uncolored, white, or pale Luster: greasy Color: variable, but usually light Chemical composition: SiO2 Specific gravity: 2.6 Comments: Opaque on thin edge; compact (dense); conchoidal fracture

1.222 FLINT Hardness: 7 Streak: uncolored, white, or pale Luster: greasy Color: variable, but usually dark Chemical composition: SiO2 Specific gravity: 2.6 Comments: Compact (dense); conchoidal fracture; translucent on thin edge 1.223 MILKY QUARTZ Hardness: 7 Streak: uncolored, white, or pale Luster: vitreous or greasy Color: milky Chemical composition: SiO2 Specific gravity: 2.65 Comments: Translucent; no cleavage; milky color due to minute liquid inclusions

7 1.224 ROSE QUARTZ Hardness: 7 Streak: uncolored, white, or pale Luster: vitreous or greasy Color: pale pink to rose red Chemical composition: SiO2 Specific gravity: 2.65 Comments: Color due to minute traces of titanium, occasionally coarsely crystalline but usually without crystalline form 1.225 JASPER Hardness: 7 Streak: pale Luster: vitreous Color: red to brown Chemical composition: SiO2 Specific gravity: 2.65 Comments: Red color caused by inclusion of hematite; conchoidal fracture. It is a cryptocrystalline quartz (submicroscopic crystals) 1.226 OLIVINE Hardness: 6.5 - 7 Streak: uncolored, white, or pale Luster: vitreous Color: olive to gray-green Chemical composition: (Mg,Fe)2SiO4 Specific gravity: 3.27 - 4.37 Comments: Granular appearance in mass; transparent variety known as peridot; fracture is visible in peridot; conchoidal 2.111 GALENA Hardness: 2.5 Streak: gray or black Luster: metallic Color: lead-gray Chemical composition: PbS Specific gravity: 7.4 - 7.6 Comments: Opaque, heavy, often in cubes, cubic cleavage in three 90° directions

8 2.112 GRAPHITE Hardness: Streak: Luster: Color: Chemical composition: Specific gravity: Comments: Has greasy feel; perfect cleavage in one direction

1 gray or black greasy gray C 2

2.121 BIOTITE (BLACK MICA) Hardness: 2.5 - 3 Streak: uncolored, white, or pale Luster: vitreous Color: dark green to black Chemical composition: (K, Mg, Fe, Al) silicate Specific gravity: 2.8 - 3.2 Comments: Basal cleavage prominent (perfect in one direction), flakes in flat sheets; flexible and elastic; transparent to translucent 2.122 CALCITE Hardness: 3 Streak: uncolored, white, or pale Luster: vitreous Color: any color, though usually white, sometimes colorless Chemical composition: CaCO3 Specific gravity: 2.71 Comments: Transparent to translucent, rhombohedral cleavage (three inclined directions not at right angles); Iceland Spar variety: clear and double refractive. 2.123 GYPSUM Hardness: 2 Streak: uncolored, white, or pale Luster: vitreous (Satin Spar variety: silky luster) Color: white or light-colored Chemical composition: CaSO4 · 2H2O Specific gravity: 2.32 Comments: Varieties: Selenite -- often clear and transparent; flexible but inelastic sheets; cleavage in three directions, one very good; Satin Spar -- silky, fibrousforming veinlets; Alabaster -- composed of aggregates of fine crystals

9 2.124 HALITE Hardness: 2.5 Streak: uncolored, white, or pale Luster: vitreous Color: colorless to white Chemical composition: NaCl Specific gravity: 2.16 Comments: Salty taste; water soluble, transparent to translucent; cubic cleavage (perfect in three directions at right angles) 2.125 MUSCOVITE (LIGHT-COLORED MICA) Hardness: 2 - 2.5 Streak: uncolored, white, or pale Luster: vitreous Color: white or light shades of green or yellow Chemical composition: KAl2(AlSi3O10)(OH)2 Specific gravity: 2.76 - 3.1 Comments: Basal cleavage prominent (perfect in one direction), flakes in flat sheets; flexible and elastic; transparent to translucent 2.126 AZURITE Hardness: 1-3 Streak: uncolored, white, or pale Luster: vitreous Color: azure blue Chemical composition: Cu3(CO3)2(OH)2 Specific gravity: 3.77 Comments: Transparent to translucent when in crystal form 2.127 SPHALERITE Hardness: 3.5 - 4 Streak: yellow brown to nearly white Luster: resinous Color: black to colorless (colorless rare) Chemical composition: ZnS Specific gravity: 3.9 - 4.1 Comments: Often shows distinct cleavage planes

10 2.128 FLUORITE Hardness: 4 Streak: uncolored, white, or pale Luster: vitreous Color: can be colorless, yellow, blue, green, or violet Chemical composition: CaF2 Specific gravity: 3.18 Comments: Transparent to translucent, cubic crystals, good cleavage in four directions 2.129 TALC Hardness: 1 Streak: uncolored, white, or pale Luster: pearly or greasy Color: white, gray-green, silver-white Chemical composition: Mg3(Si4O10)(OH)2 Specific gravity: 2.7 - 2.8 Comments: Very soft; greasy or soapy feel, foliated or compact masses; perfect cleavage in one direction, translucent on thin edge 2.211 MALACHITE Hardness: 3.5 - 4 Streak: uncolored, white, or pale Luster: vitreous (when in crystals) Color: bright green Chemical composition: Cu2CO3(OH)2 Specific gravity: 3.9 - 4.03 Comments: Silky in fibrous varieties, dull in earthy type 2.212 BAUXITE Hardness: 1-3 Streak: uncolored, white, or pale Luster: earthy Color: gray to buff or brown Chemical composition: mixture of aluminum hydroxides Specific gravity: 2.0 - 2.55 Comments: An earthy rock, composed chiefly of gibbsite, boehmite, and diaspore

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