Mineral Structures Silicates are classified on the basis of Si-O polymerism the [SiO4]4- tetrahedron Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO4]4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si2O7]6- Double tetrahedra Sorosilicates Examples: lawsonite epidote n[SiO3]2- n = 3, 4, Examples: benitoite BaTi[Si3O9] beryl Be3Al2[Si6O18] Cyclosilicates Mineral Structures Inosilicates [SiO3]2pryoxenes single chains pyroxenoids Inosilicates [Si4O11]4- Double chains amphiboles Mineral Structures Phyllosilicates [Si2O5]2- Sheets of tetrahedra micas talc clay minerals serpentine Phyllosilicates Mineral Structures Tectosilcates low-quartz [SiO2] 3-D frameworks of tetrahedra: fully polymerized quartz and the silica minerals feldspars feldspathoids zeolites Tectosilicates Nesosilicates: independent SiO4 tetrahedra b c M1 and M2 as polyhedra Olivine (100) view blue = M1 yellow = M2 Nesosilicates: Olivine (Mg,Fe)2SiO4 Olivine Occurrences: Principally in mafic and ultramafic igneous rocksTypically ~60+% of mantle source for basaltsFayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marbles Nesosilicates: Garnet Garnet: A2+3 B3+2 [SiO4]3 “Pyralspites” - B = Al Pyrope: Mg3 Al2 [SiO4]3 Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 “Ugrandites” - A = Ca Uvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3 Occurrence: Mostly metamorphic Some high-Al igneous Also in some mantle peridotites Garnet (001) view blue = Si purple = A turquoise = B Inosilicates: single chains- pyroxenes b a sinβ Diopside: CaMg [Si2O6] Where are the Si-O-Si-O chains?? Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes a sinβ b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: double chains- amphiboles b a sinβ Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 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 Amphibole Chemistry General formula: W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2 W = Na K X = Ca Na Mg Fe2+ (Mn Li) Y = Mg Fe2+ Mn Al Fe3+ 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 Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Tremolite Ca2Mg5Si8O22(OH)2 Anthophyllite Mg7Si8O22(OH)2 Actinolite Cummingtonite-grunerite Orthoamphiboles Ferroactinolite Ca2Fe5Si8O22(OH)2 Clinoamphiboles Fe7Si8O22(OH)2 Amphibole Chemistry 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: Na2 Mg3 Al2 [Si8O22] (OH)2 Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2 Sodic amphiboles are commonly blue, and often called “blue amphiboles” Amphibole Occurrences 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 pyroxene Inosilicates amphibole b a Cleavage angles can be interpreted in terms of weak bonds in M2 sites Narrow single-chain I-beams → 90o cleavages in pyroxenes while wider doublechain I-beams → 60-120o cleavages in amphiboles Phyllosilicates SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5] Apical O’s are unpolymerized and are bonded to other constituents Phyllosilicates Tetrahedral layers are bonded to octahedral layers (OH) pairs are located in center of T rings where no apical O Phyllosilicates a2 a1 Gibbsite: Al(OH)3 Layers of octahedral Al in coordination with (OH) Al3+ 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 Phyllosilicates Muscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV) T-layer - diocathedral (Al3+) layer - T-layer - K K between T - O - T groups is stronger than vdw T O T K T O T K T O T Phyllosilicates Phlogopite: K Mg3 [Si3AlO10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer - K K between T - O - T groups is stronger than vdw T O T K T O T K T O T Phyllosilicates Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (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) Tectosilicates After Swamy and Saxena (1994) J Geophys Res., 99, 11,787-11,794 Tectosilicates Low Quartz SiIV Stishovite SiVI Tectosilicates Feldspars Substitute Al3+ for Si4+ allows Na+ or K+ to be added Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite: NaAlSi3O8 [...]... amphiboles pyroxene Inosilicates amphibole b a Cleavage angles can be interpreted in terms of weak bonds in M2 sites Narrow single-chain I-beams → 90o cleavages in pyroxenes while wider doublechain I-beams → 60-120o cleavages in amphiboles Phyllosilicates SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5] Apical O’s are unpolymerized and are bonded to other constituents Phyllosilicates Tetrahedral... - T - (brucite) - T - O - T Very hydrated (OH)8, so low-temperature stability (low-T metamorphism and alteration of mafics as cool) Tectosilicates After Swamy and Saxena (1994) J Geophys Res., 99, 11,787-11,794 Tectosilicates Low Quartz SiIV Stishovite SiVI Tectosilicates Feldspars Substitute Al3+ for Si4+ allows Na+ or K+ to be added Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite: NaAlSi3O8...Inosilicates: single chains- pyroxenes (+) M2 The tetrahedral chain above the M1s is offset from that below c a (+) M1 (+) M2 The result is a monoclinic unit cell, hence clinopyroxenes e.g Diopside, Augite Inosilicates: single chains- pyroxenes Orthopyroxene c (-) M1 an orthorhombic unit cell (+) M2 Enstatite... layers (OH) pairs are located in center of T rings where no apical O Phyllosilicates a2 a1 Gibbsite: Al(OH)3 Layers of octahedral Al in coordination with (OH) Al3+ 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 Phyllosilicates Muscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV) T-layer - diocathedral... diocathedral (Al3+) layer - T-layer - K K between T - O - T groups is stronger than vdw T O T K T O T K T O T Phyllosilicates Phlogopite: K Mg3 [Si3AlO10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer - K K between T - O - T groups is stronger than vdw T O T K T O T K T O T Phyllosilicates Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6 = T - O - T - (brucite) - T - O - T - (brucite)... aegirineaugite Ca / (Ca + Na) 0.2 Ca-Tschermack’s molecule CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6 Inosilicates: double chains- amphiboles b a sinβ Tremolite: Ca2Mg5 [Si8O22] (OH)2 Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca) Inosilicates: double chains- amphiboles b a sinβ Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 Hornblende ... Be3Al2[Si6O18] Cyclosilicates Mineral Structures Inosilicates [SiO3]2pryoxenes single chains pyroxenoids Inosilicates [Si4O11]4- Double chains amphiboles Mineral Structures Phyllosilicates [Si2O5]2-...Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO4]4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si2O7]6- Double tetrahedra Sorosilicates... Phyllosilicates Mineral Structures Tectosilcates low-quartz [SiO2] 3-D frameworks of tetrahedra: fully polymerized quartz and the silica minerals feldspars feldspathoids zeolites Tectosilicates