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DePaul HON 225 - Minerals

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HON 225 Graded Lab Activity (50 points) 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. Talc, graphite 2. Gypsum (or selenite, a variety of gypsum) 2.2 Fingernail 3. Calcite 3.1 Copper 4. Fluorite 5. Apatite or glass 6. Feldspar (or microcline, a variety of feldspar) 7. Quartz 8. Topaz 9. Corundum Hardest 10. Diamond (not generally included in hardness kits) 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 appearance of a Diamond Greasy appearance of Oily glass Pearly appearance of a Pearl Resinous appearance of Yellow resins Silky appearance of Silk Vitreous appearance of 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 Bauxite4 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 directions5 1.123 PLAGIOCLASE FELDSPARS Variety: ALBITE Hardness: 6 Streak: uncolored, white, or pale Luster: vitreous Color: white or light-colored Chemical composition: Na(AlSi3O8) Specific gravity: 2.62 Comments: Two-directional cleavage Variety: LABRADORITE Hardness: 6 Streak: uncolored, white, or pale Luster: vitreous


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DePaul HON 225 - Minerals

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