1. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
the [SiO4]4- tetrahedron

2. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
[SiO4] Independent tetrahedra Nesosilicates
Examples: olivine garnet
[Si2O7] Double tetrahedra Sorosilicates
Examples: lawsonite epidote
n[SiO3]2- n = 3, 4, Cyclosilicates
Examples: benitoite BaTi[Si3O9]
beryl Be3Al2[Si6O18]

3. Mineral Structures Inosilicates
[SiO3] single chains Inosilicates [Si4O11] Double chains
pryoxenes pyroxenoids amphiboles

4. Mineral Structures Phyllosilicates
[Si2O5] Sheets of tetrahedra Phyllosilicates
micas talc clay minerals serpentine

5. Mineral Structures Tectosilcates
low-quartz
[SiO2] D frameworks of tetrahedra: fully polymerized Tectosilicates
quartz and the silica minerals feldspars feldspathoids zeolites

6. Nesosilicates: independent SiO4 tetrahedrab c
M1 and M2 as polyhedra
Olivine (100) view blue = M1 yellow = M2

7. Nesosilicates: Olivine (Mg,Fe)2SiO4 Olivine Occurrences:
Principally in mafic and ultramafic igneous rocks- Typically ~60+% of mantle source for basalts-
Fayalite in meta-ironstones and in some alkalic granitoids
Forsterite in some siliceous dolomitic marbles

8. Nesosilicates: Garnet
Garnet: A2+3 B3+2 [SiO4]3
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites
Garnet (001) view blue = Si purple = A turquoise = B

9. Inosilicates: single chains- pyroxenesb
Diopside: CaMg [Si2O6]
a sin
Where are the Si-O-Si-O chains??
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

10. Inosilicates: single chains- pyroxenesb
a sin
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

11. Inosilicates: single chains- pyroxenes
The tetrahedral chain above the M1s is offset from that below
The result is a monoclinic unit cell, hence clinopyroxenes
e.g. Diopside, Augite
(+) M2 c a
(+) M1
(+) M2

12. an orthorhombic unit cell
Inosilicates: single chains- pyroxenes
Orthopyroxene
an orthorhombic unit cell
Enstatite (Mg2Si2O6) c
(-) M1
(+) M2 a
(+) M1
(-) M2

13. 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

14. Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Wollastonite
pigeonite
1200oC
orthopyroxenes
clinopyroxenes
1000oC
Diopside
Hedenbergite
clinopyroxenes
Solvus
800oC
pigeonite
(Mg,Fe)2Si2O6
Ca(Mg,Fe)Si2O6
orthopyroxenes
Enstatite
Ferrosilite

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

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. Amphibole Chemistry 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

19. 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

20. 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”

21. Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates
Actinolite occurs in low-grade metamorphosed basic igneous rocks
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

22. Inosilicates pyroxene amphibole
Cleavage angles can be interpreted in terms of weak bonds in M2 sites
Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  o cleavages in amphiboles

23. Phyllosilicates SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]
Apical O’s are unpolymerized and are bonded to other constituents

24. Phyllosilicates Tetrahedral layers are bonded to octahedral layers
(OH) pairs are located in center of T rings where no apical O

25. Phyllosilicates a2 a1 Gibbsite: Al(OH)3
Layers of octahedral Al in coordination with (OH)
Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons
Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral

26. Phyllosilicates T O T K T O T K T O T
Muscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV)
T-layer - diocathedral (Al3+) layer - T-layer - K
K between T - O - T groups is stronger than vdw

27. Phyllosilicates T O T K T O T K T O T
Phlogopite: K Mg3 [Si3AlO10] (OH)2
T-layer - triocathedral (Mg2+) layer - T-layer - K
K between T - O - T groups is stronger than vdw

28. Phyllosilicates
Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6
= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -
Very hydrated (OH)8, so low-temperature stability (low-T metamorphism and alteration of mafics as cool)

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

30. Tectosilicates
Low Quartz Stishovite
SiIV SiVI

31. Tectosilicates Feldspars
Substitute Al3+ for Si4+ allows Na+ or K+ to be added
Substitute two Al3+ for Si4+ allows Ca2+ to be added
Albite: NaAlSi3O8