Orthosilicates Isolated tetrahedron Isolated tetrahedron Common examples Common examples Olivine, garnet, and zircon Olivine, garnet, and zircon Al 2 SiO 5 polymorphs, staurolite, topaz, titanite Al 2 SiO 5 polymorphs, staurolite, topaz, titanite Oxygen coordinate with other anions Oxygen coordinate with other anions

Olivine Composition Complete solid solution between forsterite (Mg) and fayalite (Fe) Complete solid solution between forsterite (Mg) and fayalite (Fe) Mn end members as well – rare Mn end members as well – rare Ca can be around 50% of cations, still has Fe- Mg solid solution Ca can be around 50% of cations, still has Fe- Mg solid solution Fe and Mg contents cause variations in physical properties Fe and Mg contents cause variations in physical properties Can be used to identify composition Can be used to identify composition Zoning can be common Zoning can be common

Fig Index of Refraction 2V x Birefringence Specific gravity d spacing (130) ForsteriteFayalite

Structure and composition Two distinct sites for cations: Two distinct sites for cations: M 1 = distorted, so smaller than M 2 M 1 = distorted, so smaller than M 2 M 2 = regular octahedron M 2 = regular octahedron Controls distribution of cations Controls distribution of cations M 2 only site for Ca, 1.12 Å, also may hold Fe and Mg M 2 only site for Ca, 1.12 Å, also may hold Fe and Mg M 1 and M 2 M 1 and M 2 both Fe = 0.78 Å and Mg = 0.72 Å both 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) Common Fe/Mg – bearing silicates Common Fe/Mg – bearing silicates Two common groups Two common groups Pyroxenes: single chains Pyroxenes: single chains Amphiboles: double chains Amphiboles: double chains Pyroxenes are common in MORB Pyroxenes are common in MORB Amphiboles more common on continents because of weathering Amphiboles more common on continents because of weathering

Pyroxene group General formula: XYZ 2 O 6 General formula: XYZ 2 O 6 Z/O ratio = 1/3 Z/O ratio = 1/3 Z cations usually Si, occasionally Al Z cations usually Si, occasionally Al Single chain extend along c axis Single chain extend along c axis Chains are stacked along a axis, alternating: Chains are stacked along a axis, alternating: Base faces base Base faces base Apex faces apex Apex faces apex

Fig Base facing base Apex facing Apex View down a axis View down c axis Two distinct sites, depending on location relative to chains M1 and M2 Plus tetrahedral sites

XYZ 2 O 6 Z/O ratio 1/3 Z/O ratio 1/3 X cations in M2 sites X cations in M2 sites Between bases of tetrahedrons Between bases of tetrahedrons Distorted 6- and 8- fold coordination Distorted 6- and 8- fold coordination Depends on stacking and the size of the cations Depends on stacking and the size of the cations Y cations in M1 sites Y cations in M1 sites 6-fold coordination between apical oxygen 6-fold coordination between apical oxygen

“I-beams” “I-beams” Consist of two chains connected by Y cations Consist of two chains connected by Y cations Located in M1 sites Located in M1 sites Closeness of apical oxygen and 6-fold coordination make bonds strong Closeness of apical oxygen and 6-fold coordination make bonds strong I-beam T-O-T sandwich Apex pointed at apex

I-beams held together by X cations in M2 site I-beams held together by X cations in M2 site Coordination number depends on how chains line up Coordination number depends on how chains line up 6-fold coordination gives orthorhombic symmetry – Orthopyroxenes or OPX 6-fold coordination gives orthorhombic symmetry – Orthopyroxenes or OPX 8-fold coordination gives monoclinic symmetry – Clinopyroxenes or CPX 8-fold coordination gives monoclinic symmetry – Clinopyroxenes or CPX

Crystal shapes Crystal shapes Blocky prisms, nearly square Blocky prisms, nearly square Elongate along c axis Elongate along c axis Cleavage controlled by I-beams Cleavage controlled by I-beams Cleavage typically between 87º and 93º Cleavage typically between 87º and 93º Only when viewed down the c axis Only when viewed down the c axis Mineral grain must be cut parallel to (001) Mineral grain must be cut parallel to (001)

Fig Weak planes between “I beams” = cleavage Looking 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 CC Cleavage angle depends on orientation of cut of crystal

Classification Based on two linked things Based on two linked things Composition: which cations occurs in M2 sites (facing bases of tetrahedron) Composition: which cations occurs in M2 sites (facing bases of tetrahedron) Symmetry: determined by composition Symmetry: determined by composition Most plot on ternary diagram with apices: Most plot on ternary diagram with apices: Wollastonite, Wo (Ca 2+ ) Wollastonite, Wo (Ca 2+ ) Enstatite, En (Mg 2+ ) Enstatite, En (Mg 2+ ) Ferrosilite, Fe (Fe 2+ ) Ferrosilite, Fe (Fe 2+ )

Three major groups Three major groups Orthopyroxenes (opx) – orthorhombic Orthopyroxenes (opx) – orthorhombic Ca-poor clinopyroxenes (cpx) – monoclinic Ca-poor clinopyroxenes (cpx) – monoclinic Ca-rich clinopyroxenes (cpx) – monoclinic Ca-rich clinopyroxenes (cpx) – monoclinic The amount of Ca in the mineral controls the crystal system, symmetry, and extinction angle The amount of Ca in the mineral controls the crystal system, symmetry, and extinction angle

Orthopyroxenes: Fe and Mg, but little Ca Orthopyroxenes: Fe and Mg, but little Ca Both M1 and M2 are octahedral Both M1 and M2 are octahedral Larger Fe ion more concentrated in M2 site Larger Fe ion more concentrated in M2 site These minerals are the enstatite – ferrosilite solid solution series These minerals are the enstatite – ferrosilite solid solution series

Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca clinopyroxene Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca clinopyroxene Mineral species is Pigeonite Mineral species is Pigeonite Ca restricted to M2 sites, these still mostly Fe and Mg Ca restricted to M2 sites, these still mostly Fe and Mg M1 sites all Mg and Fe M1 sites all Mg and Fe

Ca- clinopyroxene Ca- clinopyroxene Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) M2 site contains mostly Ca M2 site contains mostly Ca M1 site contains mostly Fe and Mg M1 site contains mostly Fe and Mg Most common specie is augite Most common specie is augite Al can substitute in M1 site, and for Si in tetrahedral site Al can substitute in M1 site, and for Si in tetrahedral site Na, Fe or Mg can substitute for Ca in M2 site Na, Fe or Mg can substitute for Ca in M2 site

Other common pyroxenes Other common pyroxenes Don’t fall neatly on Ca-Fe-Mg ternary diagram: Don’t fall neatly on Ca-Fe-Mg ternary diagram: Jadeite NaAlSi 2 O 6 Jadeite NaAlSi 2 O 6 Spodumene LiAlSi 2 O 6 Spodumene LiAlSi 2 O 6

Fig “Augite” Clinopyroxene Orthopyroxenes Na,Al – bearing pyroxenes Possible ranges of solid solutions

Amphibole Group Structure, composition, and classification similar to pyroxenes Structure, composition, and classification similar to pyroxenes Primary difference is they are double chains Primary difference is they are double chains Z/O ratio is 4/11 Z/O ratio is 4/11

Structure Chains extend parallel to c axis Chains extend parallel to c axis Stacked in alternating fashion like pyroxenes Stacked in alternating fashion like pyroxenes Points face points and bases face bases Points face points and bases face bases

Fig Chains are linked by sheets of octahedral sites Chains are linked by sheets of octahedral sites Three unique sites: M1, M2, and M3 Three unique sites: M1, M2, and M3 Octahedral layer between apical oxygen Octahedral layer between apical oxygen OH - O not shared with tetrahedron Shared O shared between tetrahedron

TOT layers TOT layers Two T layers (tetrahedral layers with Z ions) Two T layers (tetrahedral layers with Z ions) Intervening O layer (octahedron) with M1, M2, and M3 sites Intervening O layer (octahedron) with M1, M2, and M3 sites Form “I-beams” similar to pyroxenes Form “I-beams” similar to pyroxenes I-beam T-O-T sandwich Fig

Geometry produces six different structure sites Geometry produces six different structure sites M1, M2, and M3 between points of chains M1, M2, and M3 between points of chains M4 and A sites between bases of chains M4 and A sites between bases of chains Tetrahedral site Tetrahedral site Fig

Bonds at M4 and A sites weaker than bonds within “I-beams” Bonds at M4 and A sites weaker than bonds within “I-beams” Cleavage forms along the weak bonds Cleavage forms along the weak bonds “I-beams” wider than pyroxenes “I-beams” wider than pyroxenes Cleavage angles around 56º and 124º Cleavage angles around 56º and 124º Weak planes between “I beams” = cleavage, Looking down c axis Fig

Six cation sites: Six cation sites: M1, M2, and M3 between points of chains M1, M2, and M3 between points of chains M4 and A sites between bases of chains M4 and A sites between bases of chains Tetrahedral site Tetrahedral site Fig

Composition W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 Note: Z/O ratio 4/11 Note: Z/O ratio 4/11 Each cation fits a particular site Each cation fits a particular site W cation W cation Occurs in A site Occurs in A site Has ~10 fold coordination Has ~10 fold coordination Generally large, usually Na + Generally large, usually Na +

W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 X cations X cations Located in M4 sites Located in M4 sites Analogous to M2 sites in pyroxenes Analogous to M2 sites in pyroxenes Have 6 or 8 fold coordination depending on arrangement of chains Have 6 or 8 fold coordination depending on arrangement of chains If 8-fold, X usually Ca If 8-fold, X usually Ca If 6-fold, X usually Fe or Mg If 6-fold, X usually Fe or Mg

W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 Y cations Y cations Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Similar to M1 sites in pyroxenes Similar to M1 sites in pyroxenes Usually Mg, Fe 2+, Fe 3+, Al Usually Mg, Fe 2+, Fe 3+, Al Z cations Z cations Usually Si and Al Usually Si and Al

W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 Water – hydrous phase Water – hydrous phase Form from magma that contains water Form from magma that contains water Form from weathering of pyroxenes at surface Form from weathering of pyroxenes at surface

Composition Composition Most common amphiboles shown on ternary diagram Most common amphiboles shown on ternary diagram Wide variety of substitution, simple and coupled Wide variety of substitution, simple and coupled Divided into ortho and clino amphiboles Divided into ortho and clino amphiboles Depends on X cations in M4 site (largely amount of Ca), distorts structure Depends on X cations in M4 site (largely amount of Ca), distorts structure Reduces symmetry from orthorhombic to monoclinic Reduces symmetry from orthorhombic to monoclinic

Fig ~30% Ca exactly 2/7 of sites available for Ca Anthophylite Orthorhombic Grunerite Monoclinic TremoliteFerroactinolite W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2

Pyroxenes and Amphiboles

Pyroxenoid Group Similar to pyroxenes Similar to pyroxenes Single chains Single chains Z/O ratio 1/3 Z/O ratio 1/3 Differ in repeat distance along c axis Differ 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 pyroxenoids are kinked Difference is the pyroxenes are straight pyroxenoids are kinked Cased by larger linking cations Cased by larger linking cations

Pyroxenes Wollastonite - Ca Rhodenite - Mn

Only a few minerals Only a few minerals Most common have Ca, Mn, or Ca plus Na filling the M1 and M2 sites Most common have Ca, Mn, or Ca plus Na filling the M1 and M2 sites Wollastonite – Ca, fairly common, metamorphosed qtz and carbonate systems Wollastonite – Ca, fairly common, metamorphosed qtz and carbonate systems Rhodonite – Mn Rhodonite – Mn Pectolite – Ca and Na Pectolite – Ca and Na

Wollastonite Wollastonite Composition: Ca with some Mn and Fe substitution Composition: Ca with some Mn and Fe substitution Common in altered carbonate rocks, particularly with reaction with qtz Common in altered carbonate rocks, particularly with reaction with qtz Useful industrial mineral, replacing asbestos, also used in paints and plastics Useful industrial mineral, replacing asbestos, also used in paints and plastics