2.9 Inosilicates - Inosilicates (chain) Common Fe/Mg...

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Unformatted text preview: Inosilicates (chain) Common Fe/Mg bearing silicates Two common groups Pyroxenes are common in MORB Amphiboles more common on continents because of weathering Pyroxenes: single chains Amphiboles: double chains Pyroxene group General formula: XYZ2O6 Z/O ratio = 1/3 Z cations usually Si, occasionally Al Single chain extend along c axis Chains are stacked along a axis, alternating: Base faces base Apex faces apex View down a axis View down c axis Two distinct sites, depending on location relative to chains M1 and M2 Base facing base Apex facing Apex Fig. 141 X cations in M2 sites Y cations in M1 sites Between bases of tetrahedrons Distorted 6 and 8 fold coordination Depends on stacking and the size of the cations 6fold coordination between apical oxygen "Ibeams" Consist of two chains connected by Y cations Closeness of apical oxygen and 6fold coordination make bonds strong Located in M1 sites Apex pointed at apex Ibeam Ibeams held together by X cations in M2 site Coordination number depends on how chains line up 6fold coordination gives orthorhombic symmetry OPX 8fold coordination gives monoclinic symmetry CPX Crystallographic and optical axes align C crystallographic axis at 32 to 42 angle to the Z optical axis OPX - Orthorhombic Pigeonite CPX Monoclinic Crystal shapes Cleavage controlled by Ibeams Blocky prisms, nearly square Elongate along c axis Cleavage typically between 87 and 93 Only when viewed down the c axis Mineral grain must be cut parallel to (001) I beams tightly bonded Weak zones between faces of I beams Weak planes between "I beams" = cleavage Fig. 141 Cleavage angles are 87 and 93 Classification Based on two linked things Most plot on ternary diagram with apices: Which cations occurs in M2 sites (facing bases of tetrahedron) Cation determines symmetry Wollastonite, Wo Enstatite, En Ferrosilite, Fe Three major groups The amount of Ca in the mineral controls the extinction angle Orthopyroxenes (opx) orthorhombic LowCa clinopyroxenes (cpx) monoclinic Carich clinopyroxenes (cpx) monoclinic Orthopyroxenes: Fe and Mg, but little Ca Both M1 and M2 are octahedral Larger Fe ion more concentrated in M2 site (larger) LowCa clinopyroxene: more Ca, but no solid solution with HiCa clinopyroxene Mineral species is Pigeonite Ca restricted to M2 sites, these still mostly Fe and Mg M1 sites all Mg and Fe Ca clinopyroxene Most common specie is augite Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) M2 site contains mostly Ca M1 site contains mostly Fe and Mg Al substitutes in M1 site, and for Si in tetrahedral site Na, Fe or Mg substitutes for Ca in M2 site Other common pyroxenes Jadeite NaAlSi2O6 Spodumene LiAlSi2O6 Possible ranges of solid solutions "Augite" Clinopyroxene Orthopyroxenes Na bearing pyroxenes Fig. 142 Identification in handsample difficult Mostly based on occurrence Also color can be indicative Optical properties distinguish clino from orthopyroxenes If composition is important, need chemical analysis Geology of pyroxenes Igneous Common igneous pyroxenes: augite, pigeonite, and opx Augite most common Usually in mafic and intermediate volcanics Both intrusive and extrusive Zoning common: magma becomes enriched in Fe because of partition of Mg into crystals Exsolution common cooling allows rearrangement of Ca Requires 3 component phase diagram Exsolution mechanisms Augite original crystallization Ca substitution in M2 sites restricted As cools, Ca reorganizes Generally find exsolution lamellae of pigeonite (low Ca cpx) within host augite parallel to (001) or opx parallel to (100) Augite Matrix Opx crystallize at high T with excess Ca up to 10% Slow cooling allows Ca expelled to form exsolution of augite (hi Ca cpx) Single lamellae of augite parallel to (100) Bushveld variety S. Africa type location Opx Matrix Pigeonite grows in mafic magma Up to 10% Ca in M2 site Cooling causes Ca to expel and form augite (hiCa cpx) lamellae Single lamellae parallel to (001) Pigeonite Matrix If slow enough pigeonite converts to opx Pigeonite only preserved where cooling fast (volcanic) Slow cooling creates second set augite (hiCa cpx) parallel to (100) "Stillwater type" Opx Matrix Metamorphic Carbonate rocks, typically diopside because of Ca and Mg from calcite and dolomite Na and Ca clinopyroxenes Amphibolite common association (water) Typically restricted to high T and low P conditions Found at subduction zones (blue schist facies) Opx also in granulite facies rocks Hot enough to remove water Derived from amphiboles Sedimentary Not stable (anhydrous) Converts to clay minerals Amphibole Group Structure, composition, and classification similar to pyroxenes Primary difference is they are double chains Z/O ratio is 4/11 Structure Chains extend parallel to c axis Stacked in alternating fashion like pyroxenes Points face points and bases face bases Chains are linked by sheets of octahedral sites Three unique sites: M1, M2, and M3 Depend on location relative to Si tetrahedron Not shared O Share d O OH Fig. 1412 TOT layers Two T layers (tetrahedral layers with Z ions) Intervening O layer (octahedron) with M1, M2, and M3 sites Form "Ibeams" similar to pyroxenes Geometry produces five different structure sites M1, M2, and M3 between points of chains M4 and A sites between bases of chains Bonds at M4 and A sites weaker than bonds within "Ibeams" Cleavage forms along the weak bonds "Ibeams" wider than pyroxenes Cleavage angles around 56 and 124 Weak planes between "I beams" = cleavage Composition W01X2Y5Z8O22(OH)2 Each cation fits a particular site W cation Occurs in A site Has ~10 fold coordination Generally large, usually Na+ W01X2Y5Z8O22(OH)2 X cations Located in M4 sites Analogous to M2 sites in pyroxenes Have 6 or 8 fold coordination depending on arrangement of chains If 8fold, X usually Ca If 6fold, X usually Fe or Mg W01X2Y5Z8O22(OH)2 Y cations Z cations Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Usually Mg, Fe2+, Fe3+, Al Usually Si and Al Composition Most common amphiboles shown on ternary diagram Wide variety of substitution, simple and coupled Divided into ortho and clino amphiboles Depends on X cations in M4 site (largely amount of Ca), distorts structure Reduces symmetry from orthorhombic to monoclinic W01X2Y5Z8O22(OH)2 Tremolite Ferroactinolite ~30% Ca exactly 2/7 of sites available for Ca Grunerite Monoclinic Fig. 1413 Anthophylite Orthorhomic Pyroxenes and Amphibole s Identification Hand sample and thin section difficult Best method is association Ca and Na amphiboles commonly dark green to black, pleochroic: usually Hornblende White or pale green amphiboles usually called tremolite Geology of amphiboles Several important aspects Hydrous water part of their structure Not stable in anhydrous environments Dehydrate at high temperature High Z/O ratio (4/11) mean they should occur in Sirich rocks Generalization 1. 1. Not common in mafic and ultramafic rocks Overgrowths of amphibole on pyroxenes common Crystallize late in magmatic history; melt rich in Si and H2O 2. 2. Common in felsic to intermediate rocks 1. 2. Fe and Mg minerals either amphibole or biotite Depends on abundance of K (biotite) and Ca/Na (amphiboles) Generally amphibole tends toward intermediate rocks; biotite toward felsic 1) Amphiboles common in regional metamorphism of intermediate to mafic rocks 1) 2) 3) Note these generalities are likely to be Usually water rich from breakdown of clay and micas Metamorphic rock with abundant amphiboles called amphibolite facies At high T, amphiboles break down to pyroxenes Pyroxenoid Group Similar to pyroxenes Differ in repeat distance along c axis Single chains Z/O ratio 1/3 Cased by larger linking cations Pyroxene 2 tetrahedron repeat (5.2 ) Pyroxenoid 3 or more repeat (more than 7.3 ) Difference is the pyroxenes are straight pyroxenoids are kinked Pyroxenes Rhodenite Mn Wollastonite Ca Only a few minerals Most common Wollastonite Ca Others are Rhodonite Mn Pectolite Ca and Na Wollastonite Composition: Ca with some Mn and Fe substitution Common in altered carbonate rocks, particularly with reaction with qtz Useful industrial mineral, replacing asbestose, also used in paints and plastics ...
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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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