Framework_Silicates

Framework_Silicates - Framework Silicates two thirds of...

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Framework Silicates • two thirds of crust is framework silicates • Quartz and feldspars are most common • All will similar structures - TO 4 tetrahedral framework - T = Si or Al - each oxygen is shared with another tetrahedral - sharing of highly charged cations make structures very open • Consequences of open framework - compositional: accommodate large cations, Ca, Na, and K * charge balance maintained generally by exchange Si for Al - physical properties: specific gravity lower than most other minerals * e.g. quartz = 2.65, forsterite = 3.27 * not stable at high P * restricted to crust • Four major groups: - Silica group (SiO 2 ) - Feldspars - Feldspathoids - Zeolites Silica Group • eight natural polymorphs, and other synthetic varieties • Rare forms – High P varieties: - stishovite - coesite • Most common: - quartz - tridymite 1
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- cristobalite • Structures - quartz, tridymite, and cristobalite are all distinct - reconstructive polymorphs - each has two varieties α (low) and β (high) - two varieties are displacive polymorphs - β polymorph stable at higher T and has higher symmetry than α α-β transitions are not quenchable - β variety never found at room temperature - α variety are never stable at any P and T conditions - they represent original crystallization of β form and conversion during cooling Quartz • structure: interlocking spirals of tetrahedron - spirals may be right or left handed, entantiomorphic pair - b-quartz is hexagonal - cooling kinks a-quartz • composition: relatively pure SiO 2 - some subsitution of Fe 3+ and Al 3+ - charge balance with monovalent cations - creates numerous varieties • Varieties: (1) Microcrystalline varieties - Chert –occurs as nodules or beds in limestone, black called flint , red called jasper - Chalcedony – firbrous microcrystalline quartz, color bands or irregular color 2
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called agate (2) Coarsely crystalline varieties - amethyst – Violet or purple from trace amounts of Fe - Rose quartz – pink colored, may be caused by mineral inclusion - citrine – yellow quartz from Fe, radiation or combination - Smoky quartz – irradiation with minutes about of Al - Milky quartz – from minute fluid inclusions
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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Framework_Silicates - Framework Silicates two thirds of...

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