Inosilicates (chain)

1. 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

2. 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

3. View down c axis View down a axis Fig. 14-1 Two distinct sites, depending on location relative to chains M1 and M2 Base facing base Apex facing Apex

4. X cations in M2 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

5. “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

6. 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

7. 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

8. 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)

9. Fig. 14-1 I beams – tightly bonded Weak zones between faces of I beams Weak planes between “I beams” = cleavage Cleavage angles are 87º and 93º

10. 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

11. 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

12. Orthopyroxenes: Fe and Mg, but little Ca • Both M1 and M2 are octahedral • Larger Fe ion more concentrated in the larger M2 site

13. 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

14. 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

15. Other common pyroxenes • Jadeite NaAlSi2O6 • Spodumene LiAlSi2O6

16. Possible ranges of solid solutions Fig. 14-2 “Augite” Clinopyroxene Orthopyroxenes

17. Additional Pyroxenes • Contain Na

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

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

20. O shared with Si Fig. 14-12 • 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

21. 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

22. Geometry produces five different structure sites • M1, M2, and M3 between points of chains • M4 and A sites between bases of chains

23. 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

24. CompositionW0-1X2Y5Z8O22(OH)2 • Each cation fits a particular site • W cation • Occurs in A site • Has ~10 fold coordination • Generally large, usually Na+

25. 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

26. W0-1X2Y5Z8O22(OH)2 • Y 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

27. 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

28. W0-1X2Y5Z8O22(OH)2 Fig. 14-13 Tremolite Ferroactinolite ~30% Ca exactly 2/7 of sites available for Ca Grunerite Monoclinic Anthophylite Orthorhombic

29. Pyroxenes and Amphiboles

30. 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

31. Pyroxenes Rhodenite - Mn Wollastonite - Ca

32. Only a few minerals • Most common Wollastonite – Ca • Others are Rhodonite – Mn • Pectolite – Ca and Na

33. 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