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3.1 Framework silicates - Framework Silicates 2/3 of crust...

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Unformatted text preview: Framework Silicates 2/3 of crust is framework silicates Quartz and feldspars are most common All with similar structures TO4 tetrahedral framework T = Si or Al Each oxygen is shared with another tetrahedron Sharing of highly charged cation (Si4+) make open structures Consequences of open framework Compostional Accommodate large cations Ca, Na, and K Charge balance maintained by exchange of Al and Si Physical properties Specific gravity lower than most other minerals E.g. quartz = 2.65, olivine = 3.27 Not stable at high P Restricted to crust Four major groups: Silica group (SiO2) Feldspars Feldspathoids Zeolites Silica group Rare forms High P Eight possible polymorphs one other synthetic variety Stishovite Si in octahedral coordination Coesite Quartz, & Tridymite, & Cristobalite, & Common forms Fig. 121 Structures Reconstructive polymorphism between qtz, tridymite and cristobalite Displacive polymorphism between and varieties Tridymite and Cristobalite not stable at surface P and T Will convert to quatz Varieties of quartz Microcrystalline varieties Chert occurs as nodules or beds in limestone Chalcedony fibrous microcrystalline quartz Black version flint Red version jasper Color bands or irregular color called agate Coarsely crystalline varieties Amethyst biolet or purple from trace amounts of Fe Rose quartz pink colored, may be caused by mineral inclusions Citrine yellow quartz from Fe, radiation or combination Smoky quartz irradiation and small amounts of Al Milky quartz minute fluid inclusions Feldspar Group Three compositional end members Kfeldspar (KAlSi3O8) Ks or Or Sanidine Orthoclase Microcline Albite (NaAlSi3O8) Ab Plagioclase An and Ab Alkali Ab and Ks (Or) Anorthite (CaAl2Si2O8) An Plagioclase feldspars At high T, continuous solid solution CaAl substitutes for NaSi Generally described as fraction of An, assuming An + Ab = 100% Compositions divided into ranges and given names Albite: An0 An10 Oligoclase: An10 An30 Andesine: An30 An50 Labradorite: An50 An70 Bytownite: An70 An90 Anorthite: An90 An100 Alkali feldspars Continuous solid solution at high T K and Na same charge compensates for difference in size K and Ca don't have solid solution because different size and charge Names of compositional variations caused by solid solution in feldspar minerals Influence of temperature on the allowed solid solution of feldspar minerals Fig. 125 Origin of miscibility gap variations in solid solutions Fig. 523 Feldspathoids Similar to feldspars Common minerals Nepheline Na3K(Al4Si4O16) Leucite KAlSi2O6 Sodalite group Na8Al6Si6O24Cl2 Nepheline is the most common Chemically different from feldspars Less Si relative to Na and K Rarely found with quartz (too much Si) Found in alkalirich, silica poor igneous rocks Structurally similar to feldspars 4 and 6 member rings Linked to form framework More open than feldspars Lower specific gravity than feldspars Feldspathoid minerals compared to feldspars minerals Shaded regions are allowed solid solution Zeolites Very common group of minerals Over 40 naturally occurring varieties Over 600 synthetic ones Largest single group of silicate minerals Most commonly alteration products of basaltic and andasitic volcanic rocks Commonly too fine grained for identification from physical properties Requires xray diffraction Often considered "clay minerals' Tectosilicates, not phyllosilicates Composition Hydrated framework silicates General formula MxDy(Alx+2ySinx2yO2n) MH20 Si/Al ratio varies from 1 to 6 M usually monovalent Na or K D usually divalent Ca, Mg or others Structure Open framework of Al/Si tetrahedral Link to form open channels and voids Geometry varies from one to the other Water and cations often in voids and weakly bonded Create important properties of minerals Heulandite Fig. 1222 Occurrence All but analcime are secondary Analcime may be a primary igneous mineral, late crystallization in basalts Can be used as lowT geothermometers Variety of Uses Desiccants: hydrated, but water easily exchanges so can dessicate gasses such as CO2, freon, and organic chemicals Molecular sieves: if dehydrated, other molecules fill voids, e.g. separate N from O, purify O Water softener: Narich zeolites will remove Ca from water and replace with Na Water purification: heavy metals in acid mine drainage, isotopes from radioactive waste, contaminated soils, remove NH4 from wastewater and cat litter Soil conditioner: agriculture for water and cations, slow release of N, carrier of pesticides Feed: pigs, cattle, chicken, turkeys, improve feed efficiency, reduce waste smell, increase N retention Petroleum refining, cleaning spills, filters in paper processing Environments of formation (increasing depth of burial) Weathering with high pH Diagenesis of ash, lakes and marine Alteration from ground water Hydrothermal alteration Contact metamorphism Burial and low grade regional metamorphism ...
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