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. 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 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

8. 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)

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 (a pyroxenoid)
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 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. 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
Fig. 14-2

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. 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

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. 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+

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 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

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

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