Vitreous: Glassy appearance (70% of minerals, including quartz). • Pearly: An iridescent appearance due to reflection off of internal separation planes parallel to the mineral surface (e.g., gypsum var. selenite). • Silky: Shinier in a certain direction due to the presence of parallel mineral fibers (e.g., gypsum var. satin spar). • Greasy: A semi-shiny appearance due to the presence of a thin film of altered material of a different refractive index than the underlying fresh mineral. (e.g., serpentine) • Dull: Low reflectivity associated with materials composed of microscopic crystals (e.g., kaolinite) METALLIC LUSTERS=shiny, chrome, polished Metallic Luster Subdivisions include: • Submetallic: 20% reflectance (e.g., galena) Examine the samples listed above to become accustomed to the appearance of resinous, vitreous, pearly, silky, greasy, dull, sub-metallic, and metallic lusters. CRYSTAL FORM: The generalized shape of individual mineral crystals. -Crystal shape is a direct reflection of internal atomic structure • Equant: approximately the same size in all directions (e.g., garnet) • Blocky: equant crystals with square cross-section (e.g., galena) • Prismatic: elongate crystals with square, rectangular, or diamond cross-section (e.g., KFeldspar) • Columnar: elongate crystals approximately circular (hexagonal, octagonal) cross-section (e.g., corundum) • Platy: thick sheets stacked together (e.g., muscovite) • Bladed: elongate crystals that are flattened in one direction (e.g., kyanite) • Fibrous: composed of hair-like crystals (e.g., serpentine var. chrysotile) • Massive: no distinct grain shapes discernable (e.g., kaolinite) CLEAVAGE=regular breakage Cleavage is a form of smooth, planar breakage parallel to planes along which there are atomic structural weaknesses. Cleavage Subdivisions include: • Basal: one well-developed planar cleavage (e.g., biotite) • Prismatic: two cleavages (e.g., K-feldspar) • Cubic: three cleavages at right angles (e.g., halite) • Rhombohedral: three cleavages not at right angles (e.g., calcite) • Octohedral: four cleavages (e.g., fluorite) FRACTURE=irregular breakage Fracture Subdivisions include: • Irregular: rough surface (e.g., pyrite) • Concoidal: curved surfaces like broken glass (e.g., quartz) • Hackly: jagged and sharp (e.g., copper) • Fibrous: long fibers (e.g., chrysotile) HARDNESS Hardness is the resistance of a mineral to abrasion. A mineral’s hardness is related to the strength of its weakest bond, and minerals may have different hardnesses in different directions. For example, kyanite has a hardness of 5.5 if scratched lengthwise, but a hardness of 6.5 if scratched crosswise (numbers in Mohs scale). Since covalent bonds are stronger than ionic bonds, minerals in which covalent bonds dominate will be harder than those in which ionic bonds are prevalent. Mineral hardness is classified on the 10-point Mohs Hardness Scale, but for simplicity, minerals can be grouped as follows: • Very Soft: softer than a fingernail (H
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