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. 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

4. 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

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

8. Cleavage controlled by I-beams
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. 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

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. 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

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. 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

15. Other common pyroxenes
Jadeite NaAlSi2O6
Spodumene LiAlSi2O6

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

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. Chains are linked by sheets of octahedral sites
Shared O
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
Fig

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. 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+

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 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

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 Tremolite Ferroactinolite ~30% Ca
exactly 2/7 of sites available for Ca
Grunerite
Monoclinic
Anthophylite
Orthorhomic
Fig

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 amphibolite facies
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