1. Back to silicate structures:
nesosilicates
phyllosilicates
sorosilicates
inosilicates
cyclosilictaes
tectosilicates

2. Nesosilicates: independent SiO4 tetrahedrab c
projection
Olivine (100) view blue = M1 yellow = M2

3. Inosilicates: single chains- pyroxenes
(+) T M1
Creates an “I-beam” like unit in the structure

4. The pyroxene structure is then composed of alternating I-beams
Inosilicates: single chains- pyroxenes
(+)
(+)
The pyroxene structure is then composed of alternating I-beams
Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation
(+)
(+)
(+)
Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

5. The pyroxene structure is then composed of alternation I-beams
Inosilicates: single chains- pyroxenes
(+)
(+)
The pyroxene structure is then composed of alternation I-beams
Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation
(+)
(+)
(+)

6. The pyroxene structure is then composed of alternation I-beams
Inosilicates: single chains- pyroxenes
(-)
(-)
The pyroxene structure is then composed of alternation I-beams
Orthoopyroxenes have I-beams oriented in alternate direction in different layers
(-)
(+)
(+)

7. Inosilicates: single chains- pyroxenes
The tetrahedral chain above the M1s is thus offset from that below
The M2 slabs have a similar effect
The result is a monoclinic unit cell, hence clinopyroxenes
(+) M2 c a
(+) M1
(+) M2

8. Inosilicates: single chains- pyroxenes
Orthopyroxenes have alternating (+) and (-) I-beams
the offsets thus compensate and result in an orthorhombic unit cell c
(-) M1
(+) M2 a
(+) M1
(-) M2

9. Pyroxene Chemistry The general pyroxene formula: W1-P (X,Y)1+P Z2O6
Where
W = Ca Na
X = Mg Fe2+ Mn Ni Li
Y = Al Fe3+ Cr Ti
Z = Si Al
Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

10. Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Wollastonite Ca2Si2O6
Orthopyroxenes – solid soln between Enstatite-Ferrosilite
Clinopyroxenes – solid soln between Diopside-Hedenbergite
Diopside
CaMgSi2O6
Hedenbergite
CaFeSi2O6
clinopyroxenes
Joins – lines between end members – limited mixing away from join
pigeonite
orthopyroxenes
Enstatite
Mg2Si2O6
Ferrosilite
Fe2Si2O6

11. Orthopyroxene - Clinopyroxene
OPX and CPX have different crystal structures – results in a complex solvus between them
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Diopside
CaMgSi2O6
Hedenbergite
CaFeSi2O6
Wollastonite Ca2Si2O6
Enstatite
Mg2Si2O6
Ferrosilite
Fe2Si2O6
orthopyroxenes
clinopyroxenes
pigeonite
pigeonite
1200oC
orthopyroxenes
clinopyroxenes
1000oC
CPX
Solvus
800oC
(Mg,Fe)2Si2O6
Ca(Mg,Fe)Si2O6
OPX
OPX
CPX

12. Orthopyroxene – Clinopyroxene solvus T dependence
Complex solvus – the ‘stability’ of a particular mineral changes with T. A different mineral’s ‘stability’ may change with T differently…
OPX-CPX exsolution lamellae  Geothermometer…
CPX
CPX Di Hd Di Hd
augite
augite
Miscibility
Gap
Miscibility
Gap
Subcalcic augite
pigeonite
pigeonite
orthopyroxene
orthopyroxene En Fs En Fs
OPX
OPX
800ºC
1200ºC
Pigeonite + orthopyroxene

13. Ca-Tschermack’s molecule
Pyroxene Chemistry
“Non-quad” pyroxenes
Jadeite
Aegirine
NaAlSi2O6
NaFe3+Si2O6
0.8
Omphacite
aegirine- augite
Spodumene: LiAlSi2O6
Ca / (Ca + Na)
Ca-Tschermack’s molecule
0.2
CaAl2SiO6
Augite
Diopside-Hedenbergite
Ca(Mg,Fe)Si2O6

14. Pyroxenoids
“Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites
7.1 A
12.5 A
17.4 A
5.2 A
Pyroxene
2-tet repeat
Wollastonite
(Ca  M1)
 3-tet repeat
Rhodonite
MnSiO3
 5-tet repeat
Pyroxmangite
(Mn, Fe)SiO3
 7-tet repeat

15. Back to silicate structures:
nesosilicates
phyllosilicates
sorosilicates
inosilicates
cyclosilictaes
tectosilicates

16. Inosilicates: double chains- amphiboles
Tremolite:
Ca2Mg5 [Si8O22] (OH)2
a sin
Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)
yellow = M4 (Ca)

17. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
a sin
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

18. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
Same I-beam architecture, but the I-beams are fatter (double chains)
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe)

19. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
(+)
(+)
(+)
Same I-beam architecture, but the I-beams are fatter (double chains)
a sin
(+)
(+)
All are (+) on clinoamphiboles and alternate in orthoamphiboles
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

20. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
M1-M3 are small sites
M4 is larger (Ca)
A-site is really big
Variety of sites 
great chemical range
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

21. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
(OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra
(OH)
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

22. Amphibole Chemistry See handout for more information General formula:
W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2
W = Na K
X = Ca Na Mg Fe2+ (Mn Li)
Y = Mg Fe2+ Mn Al Fe3+ Ti
Z = Si Al
Again, the great variety of sites and sizes  a great chemical range, and hence a broad stability range
The hydrous nature implies an upper temperature stability limit

23. Amphibole Chemistry
Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)
Tremolite
Ferroactinolite
Ca2Mg5Si8O22(OH)2
Actinolite
Ca2Fe5Si8O22(OH)2
Clinoamphiboles
Cummingtonite-grunerite
Anthophyllite
Mg7Si8O22(OH)2
Fe7Si8O22(OH)2
Orthoamphiboles
Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite  gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions

24. Amphibole Chemistry Hornblende has Al in the tetrahedral site
Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member.
Sodic amphiboles
Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2
Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2
Sodic amphiboles are commonly blue, and often called “blue amphiboles”

25. Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates
Actinolite occurs in low-grade metamorphosed basic igneous rocks
Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstones
The complex solid solution called hornblende occurs in a broad variety of both igneous and metamorphic rocks
Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called “blueschist” in reference to the predominant blue sodic amphiboles
Riebeckite occurs commonly in sodic granitoid rocks

26. Inosilicates - - - - - - - - - - - -+ + + + + + a + + + + + + + + + + + + - - - - -
Clinopyroxene -
Clinoamphibole + + a + + + + - - - - - -
Orthopyroxene
Orthoamphibole
Pyroxenes and amphiboles are very similar:
Both have chains of SiO4 tetrahedra
The chains are connected into stylized I-beams by M octahedra
High-Ca monoclinic forms have all the T-O-T offsets in the same direction
Low-Ca orthorhombic forms have alternating (+) and (-) offsets

27. Inosilicates pyroxene amphibole
Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them)
Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  o cleavages in amphiboles

28. Tectosilicates
After Swamy and Saxena (1994) J. Geophys. Res., 99, 11,787-11,794.

29. Tectosilicates
Low Quartz
001 Projection Crystal Class 32

30. Tectosilicates
High Quartz at 581oC
001 Projection Crystal Class 622

31. Tectosilicates
Cristobalite
001 Projection Cubic Structure

32. Tectosilicates
Stishovite
High pressure  SiVI

33. Tectosilicates
Low Quartz Stishovite
SiIV SiVI

34. Igneous Minerals
Quartz, Feldspars (plagioclase and alkaline), Olivines, Pyroxenes, Amphiboles
Accessory Minerals – mostly in small quantities or in ‘special’ rocks
Magnetite (Fe3O4)
Ilmenite (FeTiO3)
Apatite (Ca5(PO4)3(OH,F,Cl)
Zircon (ZrSiO4)
Titanite (CaTiSiO5)
Pyrite (FeS2)
Fluorite (CaF2)