Inosilicates (chain)Inosilicates (chain) Common Fe/Mg – bearing silicatesCommon Fe/Mg – bearing silicates Two common groupsTwo common groups

Pyroxenes: single chainsPyroxenes: single chains Amphiboles: double chainsAmphiboles: double chains

Pyroxenes are common in MORBPyroxenes are common in MORB Amphiboles more common on Amphiboles more common on

continents because of weatheringcontinents because of weathering

Pyroxene groupPyroxene group

General formula: XYZGeneral formula: XYZ22OO66

Z/O ratio = 1/3Z/O ratio = 1/3 Z cations usually Si, occasionally AlZ cations usually Si, occasionally Al Single chain extend along c axisSingle chain extend along c axis Chains are stacked along a axis, Chains are stacked along a axis,

alternating:alternating: Base faces baseBase faces base Apex faces apexApex faces apex

Fig. 14-1Fig. 14-1

Base facing base

Apex facing Apex

View down a

axis

View down c

axis

Two distinct sites, depending on location relative to chainsM1 and M2

X cations in M2 sitesX cations in M2 sites Between bases of tetrahedronsBetween bases of tetrahedrons Distorted 6- and 8- fold coordinationDistorted 6- and 8- fold coordination Depends on stacking and the size of the Depends on stacking and the size of the

cationscations Y cations in M1 sitesY cations in M1 sites

6-fold coordination between apical 6-fold coordination between apical oxygenoxygen

““I-beams”I-beams” Consist of two chains connected by Y Consist of two chains connected by Y

cationscations Located in M1 sitesLocated in M1 sites

Closeness of apical oxygen and 6-fold Closeness of apical oxygen and 6-fold coordination make bonds strongcoordination make bonds strong

I-beamApex pointed at apex

I-beams held together by X cations in I-beams held together by X cations in M2 siteM2 site Coordination number depends on how Coordination number depends on how

chains line upchains line up 6-fold coordination gives 6-fold coordination gives orthoorthorhombic rhombic

symmetry - symmetry - OPXOPX 8-fold coordination gives mono8-fold coordination gives monoclinicclinic

symmetry - symmetry - CPXCPX

OPX - Orthorhombic Pigeonite – CPX - Monoclinic

Crystallographic and optical axes align

C crystallographic axis at 32 to 42º angle to the Z optical axis

Crystal shapesCrystal shapes Blocky prisms, nearly squareBlocky prisms, nearly square Elongate along c axisElongate along c axis

Cleavage controlled by I-beamsCleavage controlled by I-beams Cleavage typically between 87º and 93ºCleavage typically between 87º and 93º Only when viewed down the c axisOnly when viewed down the c axis Mineral grain must be cut parallel to Mineral grain must be cut parallel to

(001)(001)

Fig. 14-1Fig. 14-1

Weak planes between “I beams” = cleavage

Cleavage angles are 87º and 93º

I beams – tightly bonded

Weak zones between faces of

I beams

ClassificationClassification

Based on two linked thingsBased on two linked things Which cations occurs in M2 sites (facing Which cations occurs in M2 sites (facing

bases of tetrahedron)bases of tetrahedron) Cation determines symmetryCation determines symmetry

Most plot on ternary diagram with Most plot on ternary diagram with apices:apices: Wollastonite, WoWollastonite, Wo Enstatite, EnEnstatite, En Ferrosilite, FeFerrosilite, Fe

Three major groupsThree major groups Orthopyroxenes (opx) – Orthopyroxenes (opx) – orthorhombicorthorhombic Low-Ca clinopyroxenes (cpx) – Low-Ca clinopyroxenes (cpx) –

monoclinicmonoclinic Ca-rich clinopyroxenes (cpx) – Ca-rich clinopyroxenes (cpx) –

monoclinicmonoclinic The amount of Ca in the mineral The amount of Ca in the mineral

controls the extinction anglecontrols the extinction angle

Orthopyroxenes: Fe and Mg, but little Orthopyroxenes: Fe and Mg, but little CaCa Both M1 and M2 are octahedralBoth M1 and M2 are octahedral Larger Fe ion more concentrated in M2 Larger Fe ion more concentrated in M2

site (larger)site (larger)

Low-Ca clinopyroxene: more Ca, but Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca no solid solution with Hi-Ca clinopyroxeneclinopyroxene Mineral species is PigeoniteMineral species is Pigeonite Ca restricted to M2 sites, these still Ca restricted to M2 sites, these still

mostly Fe and Mgmostly Fe and Mg M1 sites all Mg and FeM1 sites all Mg and Fe

Ca- clinopyroxeneCa- clinopyroxene Diopside Mg(+Ca) to Hedenbergite Fe (+Ca)Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) M2 site contains mostly CaM2 site contains mostly Ca M1 site contains mostly Fe and MgM1 site contains mostly Fe and Mg

Most common specie is augiteMost common specie is augite Al substitutes in M1 site, and for Si in Al substitutes in M1 site, and for Si in

tetrahedral sitetetrahedral site Na, Fe or Mg substitutes for Ca in M2 siteNa, Fe or Mg substitutes for Ca in M2 site

Other common pyroxenesOther common pyroxenes Jadeite NaAlSiJadeite NaAlSi22OO66

Spodumene LiAlSiSpodumene LiAlSi22OO66

Fig. 14-2Fig. 14-2

“Augite”

Clinopyroxene

Orthopyroxenes

Na – bearing pyroxenes

Possible ranges of solid solutions

Identification in hand-sample difficultIdentification in hand-sample difficult Mostly based on occurrenceMostly based on occurrence Also color can be indicativeAlso color can be indicative Optical properties distinguish clino- from Optical properties distinguish clino- from

ortho-pyroxenesortho-pyroxenes If composition is important, need If composition is important, need

chemical analysischemical analysis

Geology of pyroxenesGeology of pyroxenes IgneousIgneous

Common igneous pyroxenes: augite, pigeonite, and Common igneous pyroxenes: augite, pigeonite, and opxopx

Augite most commonAugite most common Usually in mafic and intermediate volcanicsUsually in mafic and intermediate volcanics Both intrusive and extrusiveBoth intrusive and extrusive Zoning common: magma becomes enriched in Fe Zoning common: magma becomes enriched in Fe

because of partition of Mg into crystalsbecause of partition of Mg into crystals Requires 3 component phase diagramRequires 3 component phase diagram

Exsolution common – cooling allows rearrangement Exsolution common – cooling allows rearrangement of Caof Ca

Exsolution mechanismsExsolution mechanisms

Augite original crystallizationAugite original crystallization Ca substitution in M2 sites Ca substitution in M2 sites

restrictedrestricted As cools, Ca reorganizesAs cools, Ca reorganizes Generally find exsolution Generally find exsolution

lamellae of pigeonite (low Ca lamellae of pigeonite (low Ca cpx) within host augite parallel cpx) within host augite parallel to (001) or opx parallel to (100)to (001) or opx parallel to (100)

Augite Matrix

Opx crystallize at high T Opx crystallize at high T with excess Ca – up to 10%with excess Ca – up to 10% Slow cooling allows Ca Slow cooling allows Ca

expelled to form exsolution of expelled to form exsolution of augite (hi-Ca cpx)augite (hi-Ca cpx)

Single lamellae of augite Single lamellae of augite parallel to (100)parallel to (100)

BushveldBushveld variety – S. Africa variety – S. Africa type locationtype location

Opx Matrix

Pigeonite grows in mafic Pigeonite grows in mafic magmamagma Up to 10% Ca in M2 siteUp to 10% Ca in M2 site Cooling causes Ca to Cooling causes Ca to

expel and form augite (hi-expel and form augite (hi-Ca cpx) lamellaeCa cpx) lamellae

Single lamellae parallel to Single lamellae parallel to (001)(001)

Pigeonite Matrix

If slow enough pigeonite If slow enough pigeonite converts to opxconverts to opx Pigeonite only preserved Pigeonite only preserved

where cooling fast (volcanic)where cooling fast (volcanic) Slow cooling creates second Slow cooling creates second

set augite (hi-Ca cpx) set augite (hi-Ca cpx) parallel to (100)parallel to (100)

““StillwaterStillwater type” type”

Opx Matrix

MetamorphicMetamorphic

Carbonate rocks, typically diopside Carbonate rocks, typically diopside because of Ca and Mg from calcite and because of Ca and Mg from calcite and dolomitedolomite Amphibolite common association (water)Amphibolite common association (water)

Na and Ca clinopyroxenesNa and Ca clinopyroxenes Typically restricted to high T and low P Typically restricted to high T and low P

conditionsconditions Found at subduction zones (blue schist Found at subduction zones (blue schist

facies)facies)

Opx also in granulite facies rocksOpx also in granulite facies rocks Hot enough to remove waterHot enough to remove water Derived from amphibolesDerived from amphiboles

SedimentarySedimentary

Not stable (anhydrous)Not stable (anhydrous) Converts to clay mineralsConverts to clay minerals

Amphibole GroupAmphibole Group

Structure, composition, and Structure, composition, and classification similar to pyroxenesclassification similar to pyroxenes

Primary difference is they are double Primary difference is they are double chainschains

Z/O ratio is 4/11Z/O ratio is 4/11

StructureStructure

Chains extend parallel Chains extend parallel to c axisto c axis

Stacked in alternating Stacked in alternating fashion like pyroxenesfashion like pyroxenes

Points face points and Points face points and bases face basesbases face bases

Fig. 14-12Fig. 14-12

Chains are linked by Chains are linked by sheets of octahedral sheets of octahedral sitessites

Three unique sites: M1, Three unique sites: M1, M2, and M3M2, and M3 Depend on location Depend on location

relative to Si relative to Si tetrahedrontetrahedron

OH

Not shared O

Shared O

TOT layersTOT layers Two T layers (tetrahedral layers with Z Two T layers (tetrahedral layers with Z

ions)ions) Intervening O layer (octahedron) with Intervening O layer (octahedron) with

M1, M2, and M3 sitesM1, M2, and M3 sites Form “I-beams” similar to pyroxenesForm “I-beams” similar to pyroxenes

Geometry produces Geometry produces five different five different structure sitesstructure sites M1, M2, and M3 M1, M2, and M3

between points of between points of chainschains

M4 and A sites M4 and A sites between bases of between bases of chainschains

Bonds at M4 and A Bonds at M4 and A sites weaker than sites weaker than bonds within “I-beams”bonds within “I-beams”

Cleavage forms along Cleavage forms along the weak bondsthe weak bonds

““I-beams” wider than I-beams” wider than pyroxenespyroxenes

Cleavage angles Cleavage angles around 56º and 124ºaround 56º and 124º

Weak planes between “I beams” = cleavage

CompositionCompositionWW0-10-1XX22YY55ZZ88OO2222(OH)(OH)22

Each cation fits a particular siteEach cation fits a particular site W cationW cation

Occurs in A siteOccurs in A site Has ~10 fold coordinationHas ~10 fold coordination Generally large, usually NaGenerally large, usually Na++

WW0-10-1XX22YY55ZZ88OO2222(OH)(OH)22

X cationsX cations Located in M4 sitesLocated in M4 sites Analogous to M2 sites in pyroxenesAnalogous to M2 sites in pyroxenes Have 6 or 8 fold coordination depending Have 6 or 8 fold coordination depending

on arrangement of chainson arrangement of chains If 8-fold, X usually CaIf 8-fold, X usually Ca If 6-fold, X usually Fe or MgIf 6-fold, X usually Fe or Mg

WW0-10-1XX22YY55ZZ88OO2222(OH)(OH)22

Y cationsY cations Located in M1, M2, and M3 sites; Located in M1, M2, and M3 sites;

Octahedral cations in TOT stripsOctahedral cations in TOT strips Usually Mg, FeUsually Mg, Fe2+2+, Fe, Fe3+3+, Al, Al

Z cationsZ cations Usually Si and AlUsually Si and Al

CompositionComposition Most common amphiboles shown on ternary Most common amphiboles shown on ternary

diagramdiagram Wide variety of substitution, simple and Wide variety of substitution, simple and

coupledcoupled Divided into ortho and clino amphibolesDivided into ortho and clino amphiboles Depends on X cations in M4 site (largely Depends on X cations in M4 site (largely

amount of Ca), distorts structureamount of Ca), distorts structure Reduces symmetry from orthorhombic to Reduces symmetry from orthorhombic to

monoclinicmonoclinic

Fig. 14-13Fig. 14-13

~30% Ca exactly 2/7 of sites available for Ca

AnthophyliteOrthorhomic

GruneriteMonoclinic

Tremolite Ferroactinolite

WW0-10-1XX22YY55ZZ88OO2222(OH)(OH)22

Pyroxenes Pyroxenes and and

AmphibolAmphiboleses

IdentificationIdentification Hand sample and thin section difficultHand sample and thin section difficult Best method is associationBest method is association Ca and Na amphiboles commonly dark Ca and Na amphiboles commonly dark

green to black, pleochroic: usually green to black, pleochroic: usually HornblendeHornblende

White or pale green amphiboles usually White or pale green amphiboles usually called tremolitecalled tremolite

Geology of amphibolesGeology of amphiboles

Several important aspectsSeveral important aspects Hydrous – water part of their structureHydrous – water part of their structure Not stable in anhydrous environmentsNot stable in anhydrous environments Dehydrate at high temperatureDehydrate at high temperature High Z/O ratio (4/11) mean they should High Z/O ratio (4/11) mean they should

occur in Si-rich rocksoccur in Si-rich rocks

GeneralizationGeneralization1.1. Not common in mafic and ultramafic rocksNot common in mafic and ultramafic rocks

1.1. Crystallize late in magmatic history; melt rich in Si Crystallize late in magmatic history; melt rich in Si and Hand H22OO

2.2. Overgrowths of amphibole on pyroxenes commonOvergrowths of amphibole on pyroxenes common

2.2. Common in felsic to intermediate rocksCommon in felsic to intermediate rocks1.1. Fe and Mg minerals either amphibole or biotiteFe and Mg minerals either amphibole or biotite

2.2. Depends on abundance of K (biotite) and Ca/Na Depends on abundance of K (biotite) and Ca/Na (amphiboles)(amphiboles)

Generally amphibole tends toward intermediate rocks; Generally amphibole tends toward intermediate rocks; biotite toward felsicbiotite toward felsic

1)1) Amphiboles common in regional Amphiboles common in regional metamorphism of intermediate to mafic metamorphism of intermediate to mafic rocksrocks

1)1) Usually water rich from breakdown of clay and Usually water rich from breakdown of clay and micasmicas

2)2) Metamorphic rock with abundant amphiboles Metamorphic rock with abundant amphiboles called amphibolite faciescalled amphibolite facies

3)3) At high T, amphiboles break down to At high T, amphiboles break down to pyroxenespyroxenes

Note – these generalities are likely to be wrongNote – these generalities are likely to be wrong

Pyroxenoid GroupPyroxenoid Group Similar to pyroxenesSimilar to pyroxenes

Single chainsSingle chains Z/O ratio 1/3Z/O ratio 1/3

Differ in repeat distance along c axisDiffer in repeat distance along c axis Pyroxene – 2 tetrahedron repeat (5.2 Å)Pyroxene – 2 tetrahedron repeat (5.2 Å) Pyroxenoid – 3 or more repeat (more than 7.3 Å)Pyroxenoid – 3 or more repeat (more than 7.3 Å) Difference is the pyroxenes are straight Difference is the pyroxenes are straight

pyroxenoids are kinkedpyroxenoids are kinked Cased by larger linking cationsCased by larger linking cations

PyroxenesWollastonite - Ca

Rhodenite - Mn

Only a few mineralsOnly a few minerals Most common Wollastonite – CaMost common Wollastonite – Ca Others are Rhodonite – MnOthers are Rhodonite – Mn Pectolite – Ca and NaPectolite – Ca and Na

WollastoniteWollastonite Composition: Ca with some Mn and Fe Composition: Ca with some Mn and Fe

substitutionsubstitution Common in altered carbonate rocks, Common in altered carbonate rocks,

particularly with reaction with qtzparticularly with reaction with qtz Useful industrial mineral, replacing Useful industrial mineral, replacing

asbestose, also used in paints and asbestose, also used in paints and plasticsplastics