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 A third group is call pyroxenoids

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

Cleavage controlled by I-beams Crystal shapes Blocky prisms, nearly square in the (001) face Elongate along c axis, e.g., [001] 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 (a pyroxenoid) 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 the larger M2 site

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 Fig. 14-2

Additional Pyroxenes Contain Na

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 O shared with Si Chains are linked by sheets of octahedral sites Three unique sites: M1, M2, and M3 Depend on location relative to Si tetrahedron Also contain hydroxyl ion – hydrous mineral O not shared with Si 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 Note: Z/O ratio is 4/11, contains water

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 Orthorhombic Fig. 14-13

Pyroxenes and Amphiboles

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: pyroxenes are straight chains pyroxenoids are kinked chains Caused 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 asbestos, also used in paints and plastics