OrthosilicatesOrthosilicates Isolated tetrahedronIsolated tetrahedron Common examplesCommon examples

Olivine, garnet, and zirconOlivine, garnet, and zircon AlAl22SiOSiO55 polymorphs, staurolite, topaz, polymorphs, staurolite, topaz,

titanitetitanite Oxygen coordinate with other anionsOxygen coordinate with other anions

Olivine CompositionOlivine Composition

Complete solid solution between forsterite Complete solid solution between forsterite (Mg) and fayalite (Fe)(Mg) and fayalite (Fe) Mn end members as well – rareMn end members as well – rare Ca can be around 50% of cations, still has Fe-Ca can be around 50% of cations, still has Fe-

Mg solid solutionMg solid solution Fe and Mg contents cause variations in Fe and Mg contents cause variations in

physical propertiesphysical properties Can be used to identify compositionCan be used to identify composition Zoning can be commonZoning can be common

Fig. 16.2Fig. 16.2

Index of Refraction

2Vx

Birefringence

Specific gravity d spacing

(130)

Forsterite Fayalite

Structure and compositionStructure and composition

Two distinct sites for cations:Two distinct sites for cations: MM11 = distorted, so smaller than M = distorted, so smaller than M22

MM22 = regular octahedron = regular octahedron

Controls distribution of cationsControls distribution of cations MM22 only site for Ca, 1.12 Å, also may hold only site for Ca, 1.12 Å, also may hold

Fe and MgFe and Mg MM11 and M and M22

both Fe = 0.78 Å and Mg = 0.72 Åboth Fe = 0.78 Å and Mg = 0.72 Å

Distorted, small site, Ca will not fit

If sufficient Ca present when olivine forms, all M2 sites filled with Ca.Ca = 50 mole %Fe + Mg = 50 mole %

Ca

Mg Fe

(Plagioclase – 1553 to 1118 C)

Olivine – solid solution at high T

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

Plus tetrahedral sites

XYZXYZ22OO66

Z/O ratio 1/3Z/O ratio 1/3 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 oxygen6-fold coordination between apical oxygen

““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-beam

T-O-T sandwich

Apex 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 – OrthopyroxenesOrthopyroxenes or or OPXOPX 8-fold coordination gives mono8-fold coordination gives monoclinicclinic

symmetry – symmetry – ClinopyroxenesClinopyroxenes or or CPXCPX

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” = cleavageLooking down c axis

Cleavage angles are 87º and 93º

I beams – tightly bonded

Weak zones between faces of

I beams

OPX - Orthorhombic Pigeonite – CPX - Monoclinic

Crystallographic and optical axes align

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

C CCleavage angle depends on orientation of cut of crystal

ClassificationClassification

Based on two linked thingsBased on two linked things Composition: which cations occurs in M2 Composition: which cations occurs in M2

sites (facing bases of tetrahedron)sites (facing bases of tetrahedron) Symmetry: determined by compositionSymmetry: determined by composition

Most plot on ternary diagram with Most plot on ternary diagram with apices:apices: Wollastonite, Wo (CaWollastonite, Wo (Ca2+2+)) Enstatite, En (MgEnstatite, En (Mg2+2+)) Ferrosilite, Fe (FeFerrosilite, Fe (Fe2+2+))

Three major groupsThree major groups Orthopyroxenes (opx) – Orthopyroxenes (opx) – orthorhombicorthorhombic Ca-poor clinopyroxenes (cpx) – Ca-poor 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 crystal system, symmetry, controls the crystal system, symmetry, and extinction angleand 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

sitesite These minerals are the enstatite –These minerals are the enstatite –

ferrosilite solid solution series ferrosilite solid solution series

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 can substitute in M1 site, and for Si in Al can substitute in M1 site, and for Si in

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

Other common pyroxenesOther common pyroxenes Don’t fall neatly on Ca-Fe-Mg ternary Don’t fall neatly on Ca-Fe-Mg ternary

diagram:diagram: Jadeite NaAlSiJadeite NaAlSi22OO66

Spodumene LiAlSiSpodumene LiAlSi22OO66

Fig. 14-2Fig. 14-2

“Augite”

Clinopyroxene

Orthopyroxenes

Na,Al – bearing pyroxenes

Possible ranges of solid solutions

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: Three unique sites: M1, M2, and M3M1, M2, and M3

Octahedral layer Octahedral layer between apical between apical oxygenoxygen

OH-

O not shared with tetrahedron

Shared O shared between tetrahedron

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

I-beam

T-O-T sandwich

Fig. 14-12Fig. 14-12

Geometry produces Geometry produces six different structure six different structure sitessites 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

Tetrahedral siteTetrahedral site

Fig. 14-12Fig. 14-12

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,Looking down c axis Fig. 14-12Fig. 14-12

Six cation sites:Six cation 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

Tetrahedral siteTetrahedral site

Fig. 14-12Fig. 14-12

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

Note: Z/O ratio 4/11Note: Z/O ratio 4/11 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 Generally large, usually NaNa++

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 If 8-fold, X usually CaCa If 6-fold, X usually If 6-fold, X usually FeFe or or MgMg

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 Similar to M1 sites in pyroxenesSimilar to M1 sites in pyroxenes Usually Usually Mg, FeMg, Fe2+2+, Fe, Fe3+3+, Al, Al

Z cationsZ cations Usually Usually SiSi and and AlAl

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

Water – hydrous phaseWater – hydrous phase Form from magma that contains waterForm from magma that contains water Form from weathering of pyroxenes at Form from weathering of pyroxenes at

surfacesurface

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

AnthophyliteOrthorhombic

GruneriteMonoclinic

Tremolite Ferroactinolite

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

Pyroxenes Pyroxenes and and

AmphibolAmphiboleses

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 have Ca, Mn, or Ca Most common have Ca, Mn, or Ca

plus Na filling the M1 and M2 sitesplus Na filling the M1 and M2 sites Wollastonite – Ca, fairly common, Wollastonite – Ca, fairly common,

metamorphosed qtz and carbonate metamorphosed qtz and carbonate systemssystems

Rhodonite – MnRhodonite – 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

asbestos, also used in paints and asbestos, also used in paints and plasticsplastics