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

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Presentation on theme: "Mineral Structures Silicates are classified on the basis of Si-O polymerism the [SiO 4 ] 4- tetrahedron."— Presentation transcript:

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2 Mineral Structures Silicates are classified on the basis of Si-O polymerism the [SiO 4 ] 4- tetrahedron

3 Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO 4 ] 4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si 2 O 7 ] 6- Double tetrahedra Sorosilicates Examples: lawsonite epidote n[SiO 3 ] 2- n = 3, 4, 6 Cyclosilicates Examples: benitoite BaTi[Si 3 O 9 ] beryl Be 3 Al 2 [Si 6 O 18 ] beryl Be 3 Al 2 [Si 6 O 18 ]

4 Mineral Structures Inosilicates [SiO 3 ] 2- single chains Inosilicates [Si 4 O 11 ] 4- Double chains pryoxenes pyroxenoids amphiboles

5 Mineral Structures Phyllosilicates [Si 2 O 5 ] 2- Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine

6 Mineral Structures Tectosilcates [SiO 2 ] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates quartz and the silica minerals feldspars feldspathoids zeolites low-quartz

7 Nesosilicates: independent SiO 4 tetrahedra Olivine (100) view blue = M1 yellow = M2 b c M1 and M2 as polyhedra

8 Nesosilicates: Olivine (Mg,Fe) 2 SiO 4 Olivine Occurrences: F Principally in mafic and ultramafic igneous rocks- Typically ~60+% of mantle source for basalts - F Fayalite in meta-ironstones and in some alkalic granitoids F Forsterite in some siliceous dolomitic marbles

9 Nesosilicates: Garnet Garnet (001) view blue = Si purple = A turquoise = B Garnet: A 2+ 3 B 3+ 2 [SiO 4 ] 3 “Pyralspites” - B = Al Pyrope: Mg 3 Al 2 [SiO 4 ] 3 Almandine: Fe 3 Al 2 [SiO 4 ] 3 Spessartine: Mn 3 Al 2 [SiO 4 ] 3 “Ugrandites” - A = Ca Uvarovite: Ca 3 Cr 2 [SiO 4 ] 3 Grossularite: Ca 3 Al 2 [SiO 4 ] 3 Andradite: Ca 3 Fe 2 [SiO 4 ] 3 Occurrence: Mostly metamorphic Some high-Al igneous Also in some mantle peridotites

10 Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Diopside: CaMg [Si 2 O 6 ] b a sin  Where are the Si-O-Si-O chains??

11 Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) b a sin 

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

13 Orthopyroxene an orthorhombic unit cell Enstatite (Mg 2 Si 2 O 6 ) Inosilicates: single chains- pyroxenes c a (+) M1 (-) M1 (-) M2 (+) M2

14 Pyroxene Chemistry The general pyroxene formula: W 1-P (X,Y) 1+P Z 2 O 6 Where F W = Ca Na F X = Mg Fe 2+ Mn Ni Li F Y = Al Fe 3+ Cr Ti F Z = Si Al Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

15 Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Diopside Hedenbergite Wollastonite Enstatite Ferrosilite orthopyroxenes clinopyroxenes pigeonite (Mg,Fe) 2 Si 2 O 6 Ca(Mg,Fe)Si 2 O 6 pigeonite clinopyroxenes orthopyroxenes Solvus 1200 o C 1000 o C 800 o C

16 Pyroxene Chemistry “Non-quad” pyroxenes Jadeite NaAlSi 2 O 6 Ca(Mg,Fe)Si 2 O 6 Aegirine NaFe 3+ Si 2 O 6 Diopside-Hedenbergite Ca-Tschermack’s molecule CaAl 2 SiO 6 Ca / (Ca + Na) Omphacite aegirine- augite Augite

17 Inosilicates: double chains- amphiboles Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca) Tremolite: Ca 2 Mg 5 [Si 8 O 22 ] (OH) 2 b a sin 

18 Inosilicates: double chains- amphiboles Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 b 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

19 General formula: W 0-1 X 2 Y 5 [Z 8 O 22 ] (OH, F, Cl) 2 W = Na K X = Ca Na Mg Fe 2+ (Mn Li) Y = Mg Fe 2+ Mn Al Fe 3+ 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 Amphibole Chemistry

20 Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Amphibole Chemistry Tremolite Ca 2 Mg 5 Si 8 O 22 (OH) 2 Ferroactinolite Ca 2 Fe 5 Si 8 O 22 (OH) 2 Anthophyllite Mg 7 Si 8 O 22 (OH) 2 Fe 7 Si 8 O 22 (OH) 2 Actinolite Cummingtonite-grunerite Orthoamphiboles Clinoamphiboles

21 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: Na 2 Mg 3 Al 2 [Si 8 O 22 ] (OH) 2 Riebeckite: Na 2 Fe 2+ 3 Fe 3+ 2 [Si 8 O 22 ] (OH) 2 Sodic amphiboles are commonly blue, and often called “blue amphiboles” Amphibole Chemistry

22 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 Amphibole Occurrences

23 Inosilicates Cleavage angles can be interpreted in terms of weak bonds in M2 sites Narrow single-chain I-beams  90 o cleavages in pyroxenes while wider double- chain I-beams  o cleavages in amphiboles pyroxeneamphibole a b

24 SiO 4 tetrahedra polymerized into 2-D sheets: [Si 2 O 5 ] Apical O’s are unpolymerized and are bonded to other constituents Phyllosilicates

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

26 Phyllosilicates Gibbsite: Al(OH) 3 Layers of octahedral Al in coordination with (OH) Al 3+ 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 a1a1a1a1 a2a2a2a2

27 Phyllosilicates Muscovite: K Al 2 [Si 3 AlO 10 ] (OH) 2 (coupled K - Al IV ) T-layer - diocathedral (Al 3+ ) layer - T-layer - K TOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOT K between T - O - T groups is stronger than vdw

28 Phyllosilicates Phlogopite: K Mg 3 [Si 3 AlO 10 ] (OH) 2 T-layer - triocathedral (Mg 2+ ) layer - T-layer - K TOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOTTOTKTOTKTOT K between T - O - T groups is stronger than vdw

29 Chlorite: (Mg, Fe) 3 [(Si, Al) 4 O 10 ] (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) Phyllosilicates

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

31 Tectosilicates Low Quartz Stishovite Si IV Si VI

32 Tectosilicates Feldspars Albite: NaAlSi 3 O 8 Substitute two Al 3+ for Si 4+ allows Ca 2+ to be added Substitute Al 3+ for Si 4+ allows Na + or K + to be added


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