Geo331 lect8 - Lecture 8 Feldspars Garnets and Topaz I...

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Lecture 8: Feldspars, Garnets, and Topaz I. Feldspars: 1. Basic Data: Chemical Formula: Potassium, Sodium Alumino-silicates Mohs' hardness 6 Crystal System Monoclinic (Orthoclase), Triclinic (Plagioclase) Color (see varieties) Fracture Irregular, good cleavage in two directions Specific Gravity 2.57 (orthoclase) Refractive Index 1.52 (orthoclase) Luster: vitreous Interesting Property: Most abundant mineral in the Earth's crust Feldspars are the most abundant minerals in the Earth"s crust. They are silicate minerals and are referred to as framework silicates because the tetrahedra (of Silicon surrounded by four coordinated oxygen ions) are linked together in three dimensions to make a complex framework. Note the tetrahedra are connected through sharing of all oxygen atoms in the coordination polyhedra. A very rare clear orthoclase gem. Plagioclase Feldspars These feldspars are rich in Na and Ca. They are Sodium, Calcium, aluminum silicates. Examples include: Labradorite , Anorthite . Alkali Feldspars These minerals are sodium, potassium, aluminum silicates. Examples include: Moonstone (perhaps the most valuable variety), Amazonite , Orthoclase , Sanidine , Microcline Carved labradorite showing the play of color when the light angle changes. 2. Where are feldspars formed?
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Feldspars make up 50-60% of the volume of crustal rocks, and are found in many different rock types. They are quite common in igneous rocks, and sometimes form huge crystals (indicating that they have cooled slowly far beneath the surface of the earth). Feldspars are very important and interesting minerals but less important as a source of gemstones; feldspars are relatively susceptible to weathering and react with mild acids to produce clay. Sri Lanka, Burma, and India have produced the finest quality moonstones. Most sunstones are found in Norway, Oregon (USA), and Mexico. Facetable yellow orthoclase most often comes from Madagascar while Oregon (USA) produces brilliant labradorites. Most labradorite comes from Labrador (it's namesake), Finland, and Madagascar. There are even labradorites in Wisconsin - although not spectacular, the rocks around Wausau have produced some nice cabochons. 3. Other interesting characteristics Plagioclase feldspars  commonly twinned, twin planes produce parallel striations  on mineral's surface  gem varieties: Labradorite: - dark grey color, iridescent -  play of color is referred to as 'labradorescence'.  some wavelengths are amplified, and some are cancelled  the net result is that different spacings and orientations  produce rainbow-like effects  iridescence is mostly blue, but often with rainbow-like  appearance  Moonstone is normally set as a cabachon in rings, pins,  and pendants; it exhibits "adularescence" or "schiller" rainbow moonstones.  Sunstone and Aventurine have inclusions of Cu in the  feldspar crystal structure 
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This note was uploaded on 03/27/2008 for the course GEO 331 taught by Professor Huifang during the Spring '07 term at University of Wisconsin.

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Geo331 lect8 - Lecture 8 Feldspars Garnets and Topaz I...

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