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1 A Tour of the Rock Forming Silicates. 2 Common Silicates GroupIgneousMetamorphicSedimentary Neso (SiO 4 )Olivine group, Zircon, Topaz Garnet group,

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Presentation on theme: "1 A Tour of the Rock Forming Silicates. 2 Common Silicates GroupIgneousMetamorphicSedimentary Neso (SiO 4 )Olivine group, Zircon, Topaz Garnet group,"— Presentation transcript:

1 1 A Tour of the Rock Forming Silicates

2 2 Common Silicates GroupIgneousMetamorphicSedimentary Neso (SiO 4 )Olivine group, Zircon, Topaz Garnet group, Alumnosilicates Soro (Si 2 O 7 )EpidoteEpidote, Zoesite Cyclo (SiO 3 )Tourmaline Ino (single – SiO 3 to Si 2 O 6 ) Pyroxene group (Mg, Fe, Ca) Pyroxene group (Ca, Na) Ino (double – Si 4 O 11 ) Amphibole (hornblende) Amphibole Phyllo (Si 4 O 10 )Mica (biotite- muscovite) Mica, Chlorite, Serpentine, Talc Clays Tecto (SiO 2 )Quartz, FeldsparQuartz, Feldspar, Zeolite

3 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 4 Structural Formulas Olivine Group –Nesosilicates [SiO 4 ] –X 2 SiO 4 –X = Mg, Fe Pyroxene Group –Single chain inosilicates [SiO 3 or Si 2 O 6 ] –X 2 SiO 3 to (W,X,Y) 2 Z 2 O 6 Feldspar Group –Tecto (framework) silicates [SiO 2 ] –WZ 4 O 8 Garnet Group –Nesosilicates [SiO 4 ] –X 3 Y 2 (SiO 4 ) 3 –X = Ca, Mn, Fe, Mg –Y = Fe +3, Cr Amphibole Group –Double chain inosilicates [Si 8 O 22 ] –W 0-1 X 2 Y 5 (Z 8 O 22 )(OH, F) 2 Mica Group –Phylo (sheet) silicates [Si 4 O 10 ] –W(X,Y) 2-3 (Z 4 O 10 )(OH, F) 2

5 5 Nesosilicates: Olivine Group Common component in ultramafic-mafic igneous rocks (peridotite, gabbro, basalt) A complete solid solution series –X 2 SiO 4 –X = Mg, Fe –2:1 metals:Si Flattened crystals of Fayalite Peridot (Forsterite) Dunite xenolith Dunite inclusion in basalt

6 6 Complete solid solution: –Forsterite: Mg 2 SiO 4 –Fayalite: Fe 2 SiO 4 –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

7 7 Garnet Group (X 3 Y 2 {SiO 4 } 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: Nesosilicates: Garnet Group

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

9 9 Accessory minerals in rocks –Zircon (ZrSiO 4 ) 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 (Al 2 SiO 4 {OH,F} 2 ) Pegmatite/hydrothermal accessory mineral Hard a gem mineral Nesosilicates: Other

10 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 11 Inosilicates: Pyroxene Group Common in mafic igneous & metamorphic rocks Typically: X 2 Si 2 O 6 –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

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

13 13 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)

14 14 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 Ca 2+ –Maintaining overall charge balance requires additional substitutions Ex Na + and Si 4+ for Ca 2+ and Al 3+ In pyroxenes: Sodic pyrobole species –Coupled substitution of Na + Al 3+ for 2Ca 2+ –Jadeite->NaAlSi 2 O 6 –High pressure metamorphic minerals Inosilicates: Pyroxene Group

15 15 ChemistryClinopyroxene CaMgDiopside (Cpx) Ca(MgFe)Augite (Cpx) CaFeHedenburgite (Cpx ) Some Sodium Rich Varieties: NaFeAegirine NaAlJadeite (the gem) LiAlSpodumene General pyroxene formula: (W,X,Y) 2 Z 2 O 6 Inosilicates: Pyroxene Group

16 16 Inosilicates: Amphibole Group Double chain, hydrous silicates –W 0-1 X 2 Y 5 (Z 8 O 22 )(OH) 2 –W = Na, K –X = Ca, Na, Mn, Fe, Mg, Li –Y = Mn, Fe, Mg, Fe 3+, 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 17 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

18 18 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) 8 O 22 (OH) 2 Inosilicates: Amphibole Group

19 19 Phylosilicates: Mica Group Natural micas –K(Al,Mg,Fe) 2- 3 (AlSi 3 O 10 )(OH) 2 –Muscovite (Al micas) –Biotite (Fe-Mg micas) Miscibility gap between biotite series and muscovite Hydrous sheet silicate

20 20 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

21 21 Serpentine –Mg 3 Si 2 O 5 (OH) 4 –Low-grade alteration of olivine, pyroxene, and amphibole Talc –Mg 3 Si 4 O 10 (OH) 2 –Low-grade metamorphic rocks Chlorite –(Mg,Fe) 3 (Si,Al) 4 O 10 (OH) 2 * (Mg,Fe) 3 (OH) 6 –Greenschist facies metamorphic rocks Phylosilicates: Other Serpentine Talc

22 22 Tectosilicates: Feldspar Group The most abundant minerals in the Earths crust Framework silicates (SiO 2 ) –WZ 4 O 8 (Ca,Na,K)(Al,Si) 4 O 8 –Plagioclase Series Anorthite to albite (Ca,Na)(Al,Si) 4 O 8 –Alkali Feldspars Orthoclase to albite (Na,K)(Al,Si) 4 O 8

23 23 Plagioclase Series Essential minerals in most igneous, sedimentary, and metamorphic rocks Complete (temperature dependant) solid solution between –Albite (NaAlSi 3 O 8 ) –Anorthite (CaAl 2 Si 2 O 8 ) –Minor solid solution of K + increasing with increasing Ab content Tectosilicates: Feldspar Group

24 24 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 –(Ca x Na 1-x )(Al 1+x Si 3-x )O 8 –Ex: Ca.20 Na.80 (Al 1.2 Si 2.8 )O 8 Oligoclase

25 25 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 Tectosilicates: Feldspar Group

26 26 Tectosilicates: Quartz Group Essentially pure SiO 2 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

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

28 28 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

29 29 Minerals and Rocks FormationEssential minerals SilicateNon-silicate IgneousCrystallize from magma Olivine, pyroxene, amphibole, mica, feldspar, quartz -- SedimentaryWeathering residue Chemical precipitate Quartz, feldspar, clay Quartz (microcrystalline) -- Carbonates Halides MetamorphicAlteration of pre- existing mineral Garnet, epidote, pyroxene, amphibole, mica, quartz, feldspar Carbonates

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