sheet_silicate - Sheet Silicates Abundant and common...

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Sheet Silicates • Abundant and common minerals • Throughout upper 20 km of crust • Felsic to intermediate rocks, metamorphic, and sedimentary rocks • All are hydrous - contain H - bonded to O to form OH - • Also called “Phyllosilicates” and “Sheet Silicates” - all have flaky or platy habit • Si/O ratio of 2/5 Classification – based on structure • Two different kinds of “sheets” - T sheets: tetrahedral layers - O sheets: octahedral layers • T and O layers are joined to form sheets - the sheets are repeated in vertical direction - the layers between sheets may be vacant or filled with interlayer cations, water, or other sheets • Primary characteristic is the basal cleavage - single perfect cleavage - occurs because bonds between sheets are very weak • Octahedral Sheets - two planes of OH - anionic groups - cations are two types: * divalent (Fe 2+ or Mg 2+ ) * trivalent (Al 3+ or Fe 3+ ) 1
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• Divalent cations fill 3 of 3 sites - form trioctahedral sheets - ideal formula is Mg 3 (OH) 6 - mineral called brucite (a hydroxide – not silicate) • Trivalent cations fill 2 of 3 sites - form dioctahedral sheets - ideal formula is Al 2 (OH) 6 - mineral called gibbsite (a hydroxide – not silicate) • Charges balance - cations = 6+ - anions = 6- • Tetrahedral sheets - sheets of tetrahedrally coordinated cations - formula represented by Z 2 O 5 - Z usually Si 4+ , Al 3+ , less commonly Fe 3+ • Tetrahedron are in mesh of 6-fold rings - three oxygen on each tetrahedron shared by adjacent tetrahedron - these three are basal oxygen - the fourth, unshared oxygen is the apical oxygen • Tetrahedral layers are two oxygen thick • Tetrahedral sheet composition is Si 2 O 5 2- - may have Al 3+ or Fe 3+ substitute for Si 4+ - increases net negative charge • Symmetry of rings is hexagonal - symmetry of sheet silicates close to hexagonal 2
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- depends on arrangement of stacking • Tetrahedral and octahedral sheets always joined - apical oxygen of tetrahedral sheets formed part of octahedral sheets - oxygen replaces one of the OH - in octahedral sheets • Sheets joined in two ways: - TO layers, called 1:1 layers - TOT layers, called 2:1 layers • 1:1 layers - consists of three planes of anions - one plane is basal plane of shared tetrahedral oxygen - other side is the OH - anionic group of the octahedral sheet - middle layer is OH- anionic group with some OH - replaced by oxygen. • Can build TO layer as follows: • 2:1 layers - this layer has tetrahedral layer joined to both sides of octahedral layer - TOT structure has 4 layers of anions - both sides (outermost) are planes of basal, shared oxygen - middle planes contain original OH - from octahedral layers and apical oxygen from tetrahedron
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