1. A Tour of the Rock Forming Silicates

2. Common Silicates Group Igneous Metamorphic Sedimentary Neso (SiO4)
Olivine group, Zircon, Topaz
Garnet group, Alumnosilicates
Soro (Si2O7)
Epidote
Epidote, Zoesite
Cyclo (SiO3)
Tourmaline
Ino (single – SiO3 to Si2O6)
Pyroxene group (Mg, Fe, Ca)
Pyroxene group (Ca, Na)
Ino (double – Si4O11)
Amphibole (hornblende)
Amphibole
Phyllo (Si4O10)
Mica (biotite-muscovite)
Mica, Chlorite, Serpentine, Talc
Clays
Tecto (SiO2)
Quartz, Feldspar
Quartz, Feldspar, Zeolite

3. Structural Formulas and Silicates
The key to understanding silicate mineral groups, solid solution, and miscibility
Symbology
W = large cations, C.N. >6 (with oxygen)
Ca, Na, K
X = medium-sized, bivalent cations, C.N. = 6 (with oxygen)
Mg, Fe+2 , & Ca (sort of)
Y = medium-sized, trivalent cation, C.N. = 6 (with oxygen)
Typically Al and sometimes Fe+3
Z = small cations, C.N. = 4 (with oxygen)
Mainly Si+4, but also Al+3

4. Structural Formulas Olivine Group Pyroxene Group Feldspar Group
Nesosilicates [SiO4]
X2SiO4
X = Mg, Fe
Pyroxene Group
Single chain inosilicates [SiO3 or Si2O6]
X2SiO3 to (W,X,Y)2Z2O6
Feldspar Group
Tecto (framework) silicates [SiO2]
WZ4O8
Garnet Group
Nesosilicates [SiO4]
X3Y2 (SiO4)3
X = Ca, Mn, Fe, Mg
Y = Fe+3, Cr
Amphibole Group
Double chain inosilicates [Si8O22]
W0-1X2Y5(Z8O22)(OH, F)2
Mica Group
Phylo (sheet) silicates [Si4O10]
W(X,Y)2-3(Z4O10)(OH, F)2

5. Nesosilicates: Olivine Group
Common component in ultramafic-mafic igneous rocks (peridotite, gabbro, basalt)
A complete solid solution series
X2SiO4
X = Mg, Fe
2:1 metals:Si
Peridot (Forsterite)
Dunite inclusion in basalt
Dunite xenolith
Flattened crystals of Fayalite

6. Nesosilicates: Olivine Group
Complete solid solution:
Forsterite: Mg2SiO4
Fayalite: Fe2SiO4
Also CaMg and CaFe end members, very rare
Mg end member (Fo) crystallizes first
Mg slightly more compatible than Fe
Crystallize at high T

7. Nesosilicates: Garnet Group
Garnet Group (X3Y2 {SiO4}3)
X{A} –> divalent cations: Ca+2, Mn+2, Fe+2, Mg+2
Y{B} –> trivalent cations: Al+3, Fe+3, Cr+3
Generally high-grade (high temp and/or pressure) metamorphic rock occurrence
Gem stone of high hardness: 7-7.5

8. Nesosilicates: Garnet Group
Isomorphous minerals with some solid solution
Pyralspites (Mg, Mn, Fe)
Pyrope
Almandine
Spessartine
Ugrandites (Ca)
Uvarovite
Grossular
Andradite
uvarovite

9. Nesosilicates: Other Accessory minerals in rocks Zircon (ZrSiO4)
Commonly contains uranium and thorium (and daughter product: lead) as minor atomic substitution components
Highly useful for geochronology: radiometric dating using the unstable isotopes U & Th -> Pb
Also common as an accessory mineral in metamorphic and sedimentary (highly resistant) rocks
Topaz (Al2SiO4 {OH,F}2)
Pegmatite/hydrothermal accessory mineral
Hard  a gem mineral

10. Sorosilicates and Cyclosilicates
Sorosilicate: Epidote Group
Common in regional metamorphism (epidote-ampibolite facies)
Isostructural group with some solid solution
Cyclosilicate: Tourmaline
Common component in pegmatite
Hosts incompatible elements
Used as gemstone
Tourmaline (“watermelon”)
Epidote

11. Inosilicates: Pyroxene Group
Single Chain Inosilicate
Common in mafic igneous & metamorphic rocks
Typically: X2Si2O6
X is the divalent metal cations: Mg, Fe, (Ca)
Enstitite (Mg), Ferrosilite (Fe), Diopside (CaMg), Hedenburgite (CaFe)
1:1 metals:Si
Some amount of Ca is also possible in the Pyroxene structure
Wollastonite (Ca): not a pyroxene, rather a related mineral called a pyroxenoid

12. Inosilicates: Pyroxene Group
The Pyroxene Quadrilateral
“Real” pyroxenes
Clinopyroxenes
Diopside
Hedenburgite
AUGITE
Orthopyroxenes
Enstitite
Ferrosilite
HYPERSTHENE

13. Inosilicates: Pyroxene Group
Pyroxene quadrilateral and miscibility gaps
Gap between ortho- and clinopyroxene
Accommodation of Ca
Gap varies by temperature
Ca deficient clinopyroxene:
Augite
Ca rich orthopyroxene:
Pigeonite (hi-T only)

14. Inosilicates: Pyroxene Group
Coupled Substitution
Divalent/trivalent substitutions in tetrahedral (CN 4) and octahedral (CN 6) sites of many rock forming silicate minerals
Substitution of cations with different valences
Ex: Na+ for Ca2+
Maintaining overall charge balance requires additional substitutions
Ex Na+ and Si4+ for Ca2+ and Al3+
In pyroxenes: Sodic pyrobole species
Coupled substitution of Na+Al3+ for 2Ca2+
Jadeite->NaAlSi2O6
High pressure metamorphic minerals

15. Inosilicates: Pyroxene Group
General pyroxene formula: (W,X,Y)2Z2O6
Chemistry
Clinopyroxene
CaMg
Diopside (Cpx)
Ca(MgFe)
Augite (Cpx)
CaFe
Hedenburgite (Cpx )
Some Sodium Rich Varieties:
NaFe
Aegirine
NaAl
Jadeite (the gem)
LiAl
Spodumene

16. Inosilicates: Amphibole Group
Double chain, hydrous silicates
W0-1X2Y5(Z8O22)(OH)2
W = Na, K
X = Ca, Na, Mn, Fe, Mg, Li
Y = Mn, Fe, Mg, Fe3+, Al, Ti
Z = Al, Si
Closely related to pyroxenes
Same cations; amphiboles have water
Complete and partial solid solution
Coupled substitution
Orthorhombic and monoclinic
Ferro-actinolite

17. Inosilicates: Amphibole Group
Solid solution relationships and miscibility gap analogous to pyroxenes
Mg-Fe complete solution series
CaMg-CaFe solution series
Miscibility gap due to accommodation of Ca
NaAl coupled substitution series
Ferro-actinolite

18. Inosilicates: Amphibole Group
Hornblende: the most common (and a complicated) amphibole
“Any black amphibole”
Typical in intermediate igneous rocks
Also common high temperature metamorphic rocks
Hornblende
(K,Na)0-1(Ca,Na,Fe,Mg)2 (Mg,Fe,Al)5(Si,Al)8O22(OH)2

19. Phylosilicates: Mica Group
Hydrous sheet silicate
Natural micas
K(Al,Mg,Fe)2-3(AlSi3O10)(OH)2
Muscovite (Al micas)
Biotite (Fe-Mg micas)
Miscibility gap between biotite series and muscovite

20. Phylosilicates: Mica Group
Biotite
Essential minerals in
Igneous rocks
Muscovite: Felsic igneous rocks, Granites
Biotite: Felsic to intermediate rocks
Metamorphic rocks
Schists
pseudo-hexagonal crystalline
aggregate of muscovite

21. Phylosilicates: Other
Serpentine
Serpentine
Mg3Si2O5(OH)4
Low-grade alteration of olivine, pyroxene, and amphibole
Talc
Mg3Si4O10(OH)2
Low-grade metamorphic rocks
Chlorite
(Mg,Fe)3(Si,Al)4O10(OH)2 * (Mg,Fe)3(OH)6
Greenschist facies metamorphic rocks
Talc

22. Tectosilicates: Feldspar Group
The most abundant minerals in the Earth’s crust
Framework silicates (SiO2)
WZ4O8
(Ca,Na,K)(Al,Si)4O8
Plagioclase Series
Anorthite to albite
(Ca,Na)(Al,Si)4O8
Alkali Feldspars
Orthoclase to albite
(Na,K)(Al,Si)4O8

23. Tectosilicates: Feldspar Group
Plagioclase Series
Essential minerals in most igneous, sedimentary, and metamorphic rocks
Complete (temperature dependant) solid solution between
Albite (NaAlSi3O8)
Anorthite (CaAl2Si2O8)
Minor solid solution of K+ increasing with increasing Ab content

24. Tectosilicates: Feldspar Group
Plagioclase series: complete solid solution
Because of the similarity in ionic radius between Na+1 (0.95 Å) and Ca+2 (0.99 Å)
Isomorphous solid solution between end members
Coupled substitution
(CaxNa1-x)(Al1+xSi3-x)O8
Ex: Ca.20Na.80(Al1.2Si2.8)O8
Oligoclase

25. Tectosilicates: Feldspar Group
Alkali Feldspar: Miscibility gap
Because of dissimilarity in size between the Na+1 (0.95 Å) and K+1 (1.33 Å) complete solid solution occurs only at high temp
Wrong size “bricks” results in alkali feldspar polymorphs
Single alkali feldspars formed at high temp exsolve (unmix) at lower temp (if slowly cooled)
Orthoclase-rich-> perthite
Albite-rich-> antiperthite

26. Tectosilicates: Quartz Group
Amethyst
Essentially “pure” SiO2
Component of many felsic and intermediate igneous rocks
Not present in:
Ultramafic igneous rocks
Alkaline (feldspathoidal) igneous rocks
Common particulate residue during bedrock weathering
Common chemical precipitate in surface through hydrothermal settings
Common component of metamorphic rocks

27. Tectosilicates: Quartz Group
Silica P-T phase diagram
Silica polymorphs in P-T “space”
Hi P: coesite, stishovite
Hi T: tridymite, cristobalite

28. Tectosilicates: Quartz Group
Chalcedony: a micro- (very small) to crypto- crystalline (almost amorphous {non-crystalline}) fibrous quartz
Common precipitate in surface and near-surface conditions

29. Minerals and Rocks Formation Essential minerals Silicate Non-silicate
Igneous
Crystallize from magma
Olivine, pyroxene, amphibole, mica, feldspar, quartz --
Sedimentary
Weathering residue Chemical precipitate
Quartz, feldspar, clay Quartz (microcrystalline)
-- Carbonates
Halides
Metamorphic
Alteration of pre-existing mineral
Garnet, epidote, pyroxene, amphibole, mica, quartz, feldspar
Carbonates