Understanding Inosilicates: Pyroxenes and Amphiboles in Geology

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

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 OPX - Orthorhombic

8. 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)

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 • 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 M2 site (larger)

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 Na – bearing pyroxenes

17. Identification in hand-sample difficult • Mostly based on occurrence • Also color can be indicative • Optical properties distinguish clino- from ortho-pyroxenes • If composition is important, need chemical analysis

18. Geology of pyroxenes • Igneous • Common igneous pyroxenes: augite, pigeonite, and opx • Augite most common • Usually in mafic and intermediate volcanics • Both intrusive and extrusive • Zoning common: magma becomes enriched in Fe because of partition of Mg into crystals • Requires 3 component phase diagram • Exsolution common – cooling allows rearrangement of Ca

19. Exsolution mechanisms • Augite original crystallization • Ca substitution in M2 sites restricted • As cools, Ca reorganizes • Generally find exsolution lamellae of pigeonite (low Ca cpx) within host augite parallel to (001) or opx parallel to (100) Augite Matrix

20. Opx crystallize at high T with excess Ca – up to 10% • Slow cooling allows Ca expelled to form exsolution of augite (hi-Ca cpx) • Single lamellae of augite parallel to (100) • Bushveld variety – S. Africa type location Opx Matrix

21. Pigeonite grows in mafic magma • Up to 10% Ca in M2 site • Cooling causes Ca to expel and form augite (hi-Ca cpx) lamellae • Single lamellae parallel to (001) Pigeonite Matrix

22. If slow enough pigeonite converts to opx • Pigeonite only preserved where cooling fast (volcanic) • Slow cooling creates second set augite (hi-Ca cpx) parallel to (100) • “Stillwater type” Opx Matrix

23. Metamorphic • Carbonate rocks, typically diopside because of Ca and Mg from calcite and dolomite • Amphibolite common association (water) • Na and Ca clinopyroxenes • Typically restricted to high T and low P conditions • Found at subduction zones (blue schist facies)

24. Opx also in granulite facies rocks • Hot enough to remove water • Derived from amphiboles

25. Sedimentary • Not stable (anhydrous) • Converts to clay minerals

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

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

28. Shared O 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 Not shared O OH

29. 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

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

31. 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

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

33. 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

34. 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

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 Orthorhomic

37. Pyroxenes and Amphiboles

38. Identification • Hand sample and thin section difficult • Best method is association • Ca and Na amphiboles commonly dark green to black, pleochroic: usually Hornblende • White or pale green amphiboles usually called tremolite

39. Geology of amphiboles • Several important aspects • Hydrous – water part of their structure • Not stable in anhydrous environments • Dehydrate at high temperature • High Z/O ratio (4/11) mean they should occur in Si-rich rocks

40. Generalization • Not common in mafic and ultramafic rocks • Crystallize late in magmatic history; melt rich in Si and H2O • Overgrowths of amphibole on pyroxenes common • Common in felsic to intermediate rocks • Fe and Mg minerals either amphibole or biotite • Depends on abundance of K (biotite) and Ca/Na (amphiboles) • Generally amphibole tends toward intermediate rocks; biotite toward felsic

41. Amphiboles common in regional metamorphism of intermediate to mafic rocks • Usually water rich from breakdown of clay and micas • Metamorphic rock with abundant amphiboles called amphibolitefacies • At high T, amphiboles break down to pyroxenes Note – these generalities are likely to be wrong

42. 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

43. Pyroxenes Rhodenite - Mn Wollastonite - Ca

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

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