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A Tour of the Rock Forming Silicates
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Common SilicatesGroup 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
Quartz, Feldspar, Zeolite
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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
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Structural Formulas
• Olivine 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
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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
Flattened crystals of Fayalite
Peridot (Forsterite)
Dunite xenolith
Dunite inclusion in basalt
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• 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
Nesosilicates: Olivine Group
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• 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
Nesosilicates: Garnet Group
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• Isomorphous minerals with some solid solution
– Pyralspites(Mg, Mn, Fe)
• Pyrope• Almandine• Spessartine
– Ugrandites(Ca)
• Uvarovite • Grossular• Andradite
uvarovite
Nesosilicates: Garnet Group
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• 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
Nesosilicates: Other
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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
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Inosilicates: Pyroxene Group
• 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
• Single Chain Inosilicate
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• The Pyroxene Quadrilateral
• “Real” pyroxenes– Clinopyroxenes
• Diopside• Hedenburgite• AUGITE
– Orthopyroxenes• Enstitite• Ferrosilite• HYPERSTHENE
Inosilicates: Pyroxene Group
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• Pyroxene quadrilateral and miscibility gaps
Inosilicates: Pyroxene Group
• Gap between ortho- and clinopyroxene– Accommodation
of Ca
• Gap varies by temperature
• Ca deficient clinopyroxene:– Augite
• Ca rich orthopyroxene:– Pigeonite (hi-T
only)
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• 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
Inosilicates: Pyroxene Group
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Chemistry Clinopyroxene
CaMg Diopside (Cpx)
Ca(MgFe) Augite (Cpx)
CaFe Hedenburgite (Cpx )
Some Sodium Rich Varieties:
NaFe Aegirine
NaAl Jadeite (the gem)
LiAl Spodumene
• General pyroxene formula: (W,X,Y)2Z2O6
Inosilicates: Pyroxene Group
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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
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Inosilicates: Amphibole Group
• Solid solution relationships and miscibility gap analogous to pyroxenes
Ferro-actinolite
• Mg-Fe complete solution series
• CaMg-CaFe solution series
• Miscibility gap due to accommodation of Ca
• NaAl coupled substitution series
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• 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
Inosilicates: Amphibole Group
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Phylosilicates: Mica Group• 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
• Hydrous sheet silicate
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• Essential minerals in– Igneous rocks
• Muscovite: Felsic igneous rocks, Granites
• Biotite: Felsic to intermediate rocks
– Metamorphic rocks• Schists
Biotite
Phylosilicates: Mica Group
pseudo-hexagonal crystalline aggregate of muscovite
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• 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
Phylosilicates: OtherSerpentine
Talc
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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
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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
Tectosilicates: Feldspar Group
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• Plagioclase series: complete solid solution
Tectosilicates: Feldspar Group
– 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
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• 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
• Alkali feldspar polymorphs
• Single alkali feldspar crystal formed at high temp exsolves at lower temp (if slowly cooled)– Orthoclase-rich-> perthite– Albite-rich-> antiperthite
Tectosilicates: Feldspar Group
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Tectosilicates: Quartz Group
• 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
Amethyst
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• Silica P-T phase diagram– Silica
polymorphs in P-T “space”
– Hi P: coesite, stishovite
– Hi T: tridymite, cristobalite
Tectosilicates: Quartz Group
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• Chalcedony: a micro- (very small) to crypto- crystalline (almost amorphous {non-crystalline}) fibrous quartz
• Common precipitate in surface and near-surface conditions
Tectosilicates: Quartz Group
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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, clayQuartz (microcrystalline)
--
Carbonates
Halides
Metamorphic Alteration of pre-existing mineral
Garnet, epidote, pyroxene, amphibole, mica, quartz, feldspar
Carbonates
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