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
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