Understanding Orthosilicates: Olivine Composition & Inosilicates Overview

1. Orthosilicates • Isolated tetrahedron • Common examples • Olivine, garnet, and zircon • Al2SiO5 polymorphs, staurolite, topaz, titanite • Oxygen coordinate with other anions

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

3. Fig. 16.2 2Vx Index of Refraction Birefringence Specific gravity d spacing (130) Forsterite Fayalite

4. Structure and composition • Two distinct sites for cations: • M1 = distorted, so smaller than M2 • M2 = regular octahedron • Controls distribution of cations • M2 only site for Ca, 1.12 Å, also may hold Fe and Mg • M1 and M2 • both Fe = 0.78 Å and Mg = 0.72 Å

5. If sufficient Ca present when olivine forms, all M2 sites filled with Ca. Ca = 50 mole % Fe + Mg = 50 mole % Distorted, small site, Ca will not fit

6. Ca Fe Mg

7. Olivine – solid solution at high T (Plagioclase – 1553 to 1118 C)

8. 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

9. 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

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

11. XYZ2O6 • Z/O ratio 1/3 • 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

12. “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 I-beam T-O-T sandwich Apex pointed at apex

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

14. Crystal shapes • Blocky prisms, nearly square • Elongate along c axis • 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)

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

16. C C Cleavage angle depends on orientation of cut of crystal Crystallographic and optical axes align C crystallographic axis at 32 to 42º angle to the Z optical axis Pigeonite – CPX - Monoclinic OPX - Orthorhombic

17. Classification • Based on two linked things • Composition: which cations occurs in M2 sites (facing bases of tetrahedron) • Symmetry: determined by composition • Most plot on ternary diagram with apices: • Wollastonite, Wo (Ca2+) • Enstatite, En (Mg2+) • Ferrosilite, Fe (Fe2+)

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

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

20. 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

21. 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 can substitute in M1 site, and for Si in tetrahedral site • Na, Fe or Mg can substitute for Ca in M2 site

22. Other common pyroxenes • Don’t fall neatly on Ca-Fe-Mg ternary diagram: • Jadeite NaAlSi2O6 • Spodumene LiAlSi2O6

23. Possible ranges of solid solutions Fig. 14-2 “Augite” Clinopyroxene Orthopyroxenes Na,Al – bearing pyroxenes

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

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

26. Fig. 14-12 • Chains are linked by sheets of octahedral sites • Three unique sites: M1, M2, and M3 • Octahedral layer between apical oxygen Shared O shared between tetrahedron O not shared with tetrahedron OH-

27. 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 I-beam T-O-T sandwich Fig. 14-12

28. Geometry produces six different structure sites • M1, M2, and M3 between points of chains • M4 and A sites between bases of chains • Tetrahedral site Fig. 14-12

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

30. Six cation sites: • M1, M2, and M3 between points of chains • M4 and A sites between bases of chains • Tetrahedral site Fig. 14-12

31. CompositionW0-1X2Y5Z8O22(OH)2 • Note: Z/O ratio 4/11 • Each cation fits a particular site • W cation • Occurs in A site • Has ~10 fold coordination • Generally large, usually Na+

32. 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

33. W0-1X2Y5Z8O22(OH)2 • Y cations • Located in M1, M2, and M3 sites; Octahedral cations in TOT strips • Similar to M1 sites in pyroxenes • Usually Mg, Fe2+, Fe3+, Al • Z cations • Usually Si and Al

34. W0-1X2Y5Z8O22(OH)2 • Water – hydrous phase • Form from magma that contains water • Form from weathering of pyroxenes at surface

35. 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

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

37. Pyroxenes and Amphiboles

38. 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 is the pyroxenes are straight pyroxenoids are kinked • Cased by larger linking cations

39. Pyroxenes Rhodenite - Mn Wollastonite - Ca

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

41. 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