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Mineral Structures Silicates are classified on the basis of Si-O polymerism The culprit: 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 culprit: 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 culprit: 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 n[SiO 3 ] 2- n = 3, 4, 6 Cyclosilicates Examples: benitoite BaTi[Si 3 O 9 ] axinite Ca 3 Al 2 BO 3 [Si 4 O 12 ]OH axinite Ca 3 Al 2 BO 3 [Si 4 O 12 ]OH beryl Be 3 Al 2 [Si 6 O 18 ] beryl Be 3 Al 2 [Si 6 O 18 ]

4 Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO 3 ] 2- single chains Inosilicates [Si 4 O 11 ] 4- Double tetrahedra pryoxenes pyroxenoids amphiboles

5 Mineral Structures Silicates are classified on the basis of Si-O polymerism [Si 2 O 5 ] 2- Sheets of tetrahedra Phyllosilicates micas talc clay minerals serpentine

6 Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO 2 ] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates quartz and the silica minerals feldspars feldspathoids zeolites low-quartz

7 Mineral Structures Nesosilicates: independent SiO 4 tetrahedra

8 Olivine (100) view blue = M1 yellow = M2 b c projection

9 b c perspective Nesosilicates: independent SiO 4 tetrahedra

10 Olivine (001) view blue = M1 yellow = M2 M1 in rows and share edges M2 form layers in a-c that share corners Some M2 and M1 share edges b a Nesosilicates: independent SiO 4 tetrahedra

11 Olivine (100) view blue = M1 yellow = M2 b c M1 and M2 as polyhedra

12 Nesosilicates: independent SiO 4 tetrahedra Olivine Occurrences: F Principally in mafic and ultramafic igneous and meta- igneous rocks F Fayalite in meta-ironstones and in some alkalic granitoids F Forsterite in some siliceous dolomitic marbles Monticellite CaMgSiO 4 Ca  M2 (larger ion, larger site) High grade metamorphic siliceous carbonates

13 Nesosilicates: independent SiO 4 tetrahedra 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

14 Nesosilicates: independent SiO 4 tetrahedra 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 Pyralspites in meta-shales Ugrandites in meta-carbonates Some high-Al igneous Also in some mantle peridotites a1a1a1a1 a2a2a2a2 a3a3a3a3

15 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??

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

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

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

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

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

21 Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Perspective view

22 Inosilicates: single chains- pyroxenes Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) SiO 4 as polygons (and larger area) IV slab VI slab b a sin 

23 Inosilicates: single chains- pyroxenes M1 octahedron

24 Inosilicates: single chains- pyroxenes M1 octahedron

25 Inosilicates: single chains- pyroxenes M1 octahedron (+) type by convention (+)

26 Inosilicates: single chains- pyroxenes M1 octahedron This is a (-) type (-)

27 Inosilicates: single chains- pyroxenes TM1T Creates an “I-beam” like unit in the structure.

28 Inosilicates: single chains- pyroxenes TM1T Creates an “I-beam” like unit in the structure (+)

29 The pyroxene structure is then composed of alternating I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation (+) (+) (+) (+)(+) Inosilicates: single chains- pyroxenes Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

30 The pyroxene structure is then composed of alternation I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation (+) (+) (+) Inosilicates: single chains- pyroxenes (+) (+)

31 Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms Inosilicates: single chains- pyroxenes

32 The tetrahedral chain above the M1s is thus offset from that below The M2 slabs have a similar effect The result is a monoclinic unit cell, hence clinopyroxenes Inosilicates: single chains- pyroxenes c a (+) M1 (+) M2

33 Orthopyroxenes have alternating (+) and (-) I-beams the offsets thus compensate and result in an orthorhombic unit cell This also explains the double a cell dimension and why orthopyroxenes have {210} cleavages instead of {110) as in clinopyroxenes (although both are at 90 o ) Inosilicates: single chains- pyroxenes c a (+) M1 (-) M1 (-) M2 (+) M2

34 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

35 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

36 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 CaAl2SiO 6 Ca / (Ca + Na) Omphacite aegirine- augite Augite Spodumene: LiAlSi 2 O 6

37 Pyroxenoids “Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites Wollastonite (Ca  M1) (Ca  M1)  3-tet repeat Rhodonite MnSiO 3  5-tet repeat Pyroxmangite (Mn, Fe)SiO 3 (Mn, Fe)SiO 3  7-tet repeat Pyroxene 2-tet repeat 7.1 A 12.5 A 17.4 A 5.2 A

38 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 

39 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

40 Inosilicates: double chains- amphiboles Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 Same I-beam architecture, but the I-beams are fatter (double chains)

41 Inosilicates: double chains- amphiboles b a sin  (+) (+) (+) (+) (+) Same I-beam architecture, but the I-beams are fatter (double chains) All are (+) on clinoamphiboles and alternate in orthoamphiboles 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 Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2

42 Inosilicates: double chains- amphiboles 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 Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 M1-M3 are small sites M4 is larger (Ca) A-site is really big Variety of sites  great chemical range

43 Inosilicates: double chains- amphiboles 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 Hornblende: (Ca, Na) 2-3 (Mg, Fe, Al) 5 [(Si,Al) 8 O 22 ] (OH) 2 (OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra (OH)

44 See handout for more information 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

45 Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Amphibole Chemistry Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite  gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions 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

46 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

47 Tremolite (Ca-Mg) occurs in meta-carbonates Actinolite occurs in low-grade metamorphosed basic igneous rocks Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstones The complex solid solution called hornblende occurs in a broad variety of both igenous 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 Riebeckite occurs commonly in sodic granitoid rocks Amphibole Occurrences

48 Inosilicates Pyroxenes and amphiboles are very similar: F Both have chains of SiO 4 tetrahedra F The chains are connected into stylized I-beams by M octahedra F High-Ca monoclinic forms have all the T-O-T offsets in the same direction F Low-Ca orthorhombic forms have alternating (+) and (-) offsets a a Clinopyroxene Orthopyroxene Orthoamphibole Clinoamphibole

49 Inosilicates Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them) Narrow single-chain I-beams  90 o cleavages in pyroxenes while wider double- chain I-beams  o cleavages in amphiboles pyroxeneamphibole a b

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

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

52 Octahedral layers can be understood by analogy with hydroxides Phyllosilicates Brucite: Mg(OH) 2 Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals bonds c

53 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

54 Phyllosilicates Kaolinite: Al 2 [Si 2 O 5 ] (OH) 4 T-layers and diocathedral (Al 3+ ) layers (OH) at center of T-rings and fill base of VI layer  Yellow = (OH) TO-TO-TOTO-TO-TOTO-TO-TOTO-TO-TO vdw vdw weak van der Waals bonds between T-O groups

55 Phyllosilicates Serpentine: Mg 3 [Si 2 O 5 ] (OH) 4 T-layers and triocathedral (Mg 2+ ) layers (OH) at center of T-rings and fill base of VI layer  Yellow = (OH) TO-TO-TOTO-TO-TOTO-TO-TOTO-TO-TO vdw vdw weak van der Waals bonds between T-O groups

56 Serpentine Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999). Antigorite maintains a sheet-like form by alternating segments of opposite curvature Chrysotile does not do this and tends to roll into tubes

57 Serpentine The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates S = serpentine T = talc Nagby and Faust (1956) Am. Mineralogist 41, Veblen and Busek, 1979, Science 206,

58 Phyllosilicates Pyrophyllite: Al 2 [Si 4 O 10 ] (OH) 2 T-layer - diocathedral (Al 3+ ) layer - T-layer TOT-TOT-TOTTOT-TOT-TOTTOT-TOT-TOTTOT-TOT-TOT vdw vdw weak van der Waals bonds between T - O - T groups Yellow = (OH)

59 Phyllosilicates Talc: Mg 3 [Si 4 O 10 ] (OH) 2 T-layer - triocathedral (Mg 2+ ) layer - T-layer TOT-TOT-TOTTOT-TOT-TOTTOT-TOT-TOTTOT-TOT-TOT vdw vdw weak van der Waals bonds between T - O - T groups Yellow = (OH)

60 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

61 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

62 A Summary of Phyllosilicate Structures Phyllosilicates Fig Klein and Hurlbut Manual of Mineralogy, © John Wiley & Sons

63 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

64 Why are there single-chain-, double-chain-, and sheet-polymer types, and not triple chains, quadruple chains, etc?? “Biopyriboles”

65 It turns out that there are some intermediate types, predicted by J.B. Thompson and discovered in 1977 Veblen, Buseck, and Burnham Cover of Science: anthophyllite (yellow) reacted to form chesterite (blue & green) and jimthompsonite (red) Streaked areas are highly disordered “Biopyriboles” Cover of Science, October 28, 1977 © AAAS

66 HRTEM image of anthophyllite (left) with typical double-chain width Jimthompsonite (center) has triple-chains Chesterite is an ordered alternation of double- and triple-chains anthophyllite jimthompsonite chesterite Fig. 6, Veblen et al (1977) Science 198 © AAAS

67 Disordered structures show 4-chain widths and even a 7-chain width Obscures the distinction between pyroxenes, amphiboles, and micas (hence the term biopyriboles: biotite-pyroxene-amphibole) “Biopyriboles” Fig. 7, Veblen et al (1977) Science 198 © AAAS

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

69 Tectosilicates Low Quartz 001 Projection Crystal Class 32

70 Tectosilicates High Quartz at 581 o C 001 Projection Crystal Class 622

71 Tectosilicates Cristobalite 001 Projection Cubic Structure

72 Tectosilicates Stishovite High pressure  Si VI

73 Tectosilicates Low Quartz Stishovite Si IV Si VI

74 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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