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

Two distinct sites, depending on location relative to chains View down c axis View down a 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 Y cations in M1 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

Pigeonite – CPX - Monoclinic OPX - Orthorhombic Crystallographic and optical axes align C crystallographic axis at 32 to 42º angle to the Z optical axis Pigeonite – CPX - Monoclinic OPX - Orthorhombic

Cleavage controlled by I-beams 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 planes between “I beams” = cleavage Cleavage angles are 87º and 93º Weak zones between faces of I beams 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

The amount of Ca in the mineral controls the extinction angle Three major groups 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 species is Pigeonite Ca restricted to M2 sites, these still mostly Fe and Mg M1 sites all Mg and Fe

Most common specie is augite 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 of solid solutions “Augite” Clinopyroxene 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 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 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 (hi-Ca 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 (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 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 Shared O 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 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” 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 6 or 8 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 Z 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 Ferroactinolite ~30% Ca exactly 2/7 of sites available for Ca Grunerite Monoclinic Anthophylite Orthorhomic 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 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

Amphiboles common in regional metamorphism of intermediate to mafic rocks 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 Note – these generalities are likely to be wrong

Pyroxenoid Group Similar to pyroxenes Single chains Z/O ratio 1/3 Differ in repeat distance along c axis Pyroxene – 2 tetrahedron repeat (5.2 Å) Pyroxenoid – 3 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