Pyroxenes are common in MORB

Inosilicates (chain)   Common Fe/Mg – bearing silicates Two common groups     Pyroxenes: single chains Amphiboles: double chains Pyroxenes are common in MORB Amphiboles more common on continents because of weathering 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 14-1  X cations in M2 sites     Between bases of tetrahedrons Distorted 6- and 8- fold coordination Depends on stacking and the size of the cations Y cations in M1 sites  6-fold coordination between apical oxygen  “I-beams”  Consist of two chains connected by Y cations   Located in M1 sites Closeness of apical oxygen and 6-fold coordination make bonds strong Apex pointed at apex I-beam  I-beams held together by X cations in M2 site    Coordination number depends on how chains line up 6-fold coordination gives orthorhombic symmetry - OPX 8-fold coordination gives monoclinic symmetry - CPX Crystallographic and optical axes align C crystallographic axis at 32 to 42º angle to the Z optical axis OPX - Orthorhombic Pigeonite – CPX Monoclinic  Crystal shapes    Blocky prisms, nearly square Elongate along c axis Cleavage controlled by I-beams    Cleavage typically between 87º and 93º 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 Cleavage angles are 87º and 93º Weak planes between “I beams” = cleavage Fig 14-1 Classification  Based on two linked things    Which cations occurs in M2 sites (facing bases of tetrahedron) Cation determines symmetry Most plot on ternary diagram with apices:    Wollastonite, Wo Enstatite, En Ferrosilite, Fe     Bonds at M4 and A sites weaker than bonds within “I-beams” Cleavage forms along the weak bonds “I-beams” wider than pyroxenes Cleavage angles around 56º and 124º Weak planes between “I beams” = cleavage Composition W0-1X2Y5Z8O22(OH)2   Each cation fits a particular site W cation    Occurs in A site Has ~10 fold coordination Generally large, usually Na+ W0-1X2Y5Z8O22(OH)2  X cations      Located in M4 sites Analogous to M2 sites in pyroxenes Have or fold coordination depending on arrangement of chains If 8-fold, X usually Ca If 6-fold, X usually Fe or Mg W0-1X2Y5Z8O22(OH)2  Y cations    Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Usually Mg, Fe2+, Fe3+, Al Z cations  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 W0-1X2Y5Z8O22(OH)2 Tremolite Anthophylite Orthorhomic Ferroactinolite ~30% Ca exactly 2/7 of sites available for Ca Grunerite Monoclinic Fig 14-13 Pyroxenes and Amphiboles  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 Si-rich rocks  Generalization Not common in mafic and ultramafic rocks 2 Crystallize late in magmatic history; melt rich in Si and H2O Overgrowths of amphibole on pyroxenes common Common in felsic to intermediate rocks  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 Usually water rich from breakdown of clay and micas 2) Metamorphic rock with abundant amphiboles called amphibolite facies 3) At high T, amphiboles break down to pyroxenes Note – these generalities are likely to be wrong 1) Pyroxenoid Group  Similar to pyroxenes    Differ in repeat distance along c axis     Single chains Z/O ratio 1/3 Pyroxene – tetrahedron repeat (5.2 Å) Pyroxenoid – or more repeat (more than 7.3 Å) Difference is the pyroxenes are straight pyroxenoids are kinked Cased by larger linking cations 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 [...]... Orthopyroxenes (opx) – orthorhombic Low-Ca clinopyroxenes (cpx) – monoclinic Ca-rich clinopyroxenes (cpx) – monoclinic The amount of Ca in the mineral controls the extinction angle  Orthopyroxenes: Fe and Mg, but little Ca   Both M1 and M2 are octahedral Larger Fe ion more concentrated in M2 site (larger)  Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca clinopyroxene    Mineral... Orthopyroxenes Na – bearing pyroxenes Fig 14-2  Identification in hand-sample difficult     Mostly based on occurrence Also color can be indicative Optical properties distinguish clino- from ortho -pyroxenes 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... 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... Both intrusive and extrusive Zoning common: magma becomes enriched in Fe because of partition of Mg into crystals   Requires 3 component phase diagram Exsolution common – cooling allows rearrangement of Ca 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...  Ca- clinopyroxene     Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) M2 site contains mostly Ca M1 site contains mostly Fe and Mg Most common specie is augite   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 solid solutions of “Augite” Clinopyroxene Orthopyroxenes... tetrahedron Not shared O Shared O OH Fig 14-12  TOT layers    Two T layers (tetrahedral layers with Z ions) Intervening O layer (octahedron) with M1, M2, and M3 sites Form “I-beams” 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 “I-beams”... “I-beams” wider than pyroxenes Cleavage angles around 56º and 124º Weak planes between “I beams” = cleavage Composition W0-1X2Y5Z8O22(OH)2   Each cation fits a particular site W cation    Occurs in A site Has ~10 fold coordination Generally large, usually Na+ W0-1X2Y5Z8O22(OH)2  X cations      Located in M4 sites Analogous to M2 sites in pyroxenes Have 6 or 8 fold coordination depending on arrangement... chains If 8-fold, X usually Ca If 6-fold, X usually Fe or Mg W0-1X2Y5Z8O22(OH)2  Y cations    Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Usually Mg, Fe2+, Fe3+, Al Z cations  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. .. crystallize at high T with excess Ca – up to 10%    Slow cooling allows Ca expelled to form exsolution of augite (hiCa 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 (hi-Ca cpx) lamellae Single lamellae parallel to (001) Pigeonite Matrix  If slow... cooling fast (volcanic) Slow cooling creates second set augite (hi-Ca cpx) parallel to (100) “Stillwater type” Opx Matrix Metamorphic  Carbonate rocks, typically diopside because of Ca and Mg from calcite and dolomite   Amphibolite common association (water) Na and Ca clinopyroxenes   Typically restricted to high T and low P conditions Found at subduction zones (blue schist facies)  Opx also in ... (cpx) – monoclinic Ca-rich clinopyroxenes (cpx) – monoclinic The amount of Ca in the mineral controls the extinction angle  Orthopyroxenes: Fe and Mg, but little Ca   Both M1 and M2 are octahedral... 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... Orthopyroxenes Na – bearing pyroxenes Fig 14-2  Identification in hand-sample difficult     Mostly based on occurrence Also color can be indicative Optical properties distinguish clino-