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Silicates

Silicates. JD Price. Silicate Structure. Silicate Structure. (SiO2). Shortcuts to mineral formulae. W cations with 8- (Ca 2+ , Fe 2+ , Mn 2+ , Na + ) to 12-fold coordination (K + , Ba 2+ ) X divalent cations in 6-fold coordination (Ca 2+ , Mg 2+ , Mn 2+ , Fe 2+ )

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Silicates

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  1. Silicates JD Price

  2. Silicate Structure

  3. Silicate Structure

  4. (SiO2)

  5. Shortcuts to mineral formulae W cations with 8- (Ca 2+, Fe 2+, Mn2+, Na+) to 12-fold coordination (K+, Ba 2+) X divalent cations in 6-fold coordination (Ca 2+, Mg 2+, Mn 2+, Fe2+) Y tri- or quadrivalent in 6-fold coordination (Al3+, Fe3+, Ti4+) Z tri- or quadrivalent in 4-fold coordination (Al3+, Fe3+, Si4+, Ti4+)

  6. Components of the earth • Continental crust - felsic minerals (e.g. quartz and feldspar) • Oceanic crust - felsic + mafic (e.g. feldspar, pyroxene, hornblende) • Mantle - mafic minerals (e.g. olivine and pyroxene)

  7. NesosilicatesOlivine Forsterite Mg2SiO4 Fayalite Fe2SiO4 Image from mineral.galleries.com

  8. Solid solution With continued crystallization, what happens to the composition of the olivine s.s. and the melt? Image from Klein and Hurlbut, 1985

  9. Olivine structure Image from Klein and Hurlbut, 1985

  10. Garnet X3Y2(ZO4)3 Spessartine Mn3Al2(SiO4)3 Almandine Fe3Al2(SiO4)3 Pyrope Mg3Al2(SiO4)3 Grossular Ca3Al2(SiO4)3 Uvarolite Ca3Cr2(SiO4)3 Andradite Ca3Fe2(SiO4)3 Image from mineral.galleries.com

  11. Habits Image from Klein and Hurlbut, 1985

  12. Garnet Structure Image from Klein and Hurlbut, 1985

  13. Other nesosilicates and subsaturates Aluminosilicate Al2SiO5 {AlAl(SiO4)O} Andalusite - Sillimanite - Kyanite Staurolite (Fe, Mg,Zn)2Al9[(Si,Al)4O16]O6(OH)2 Zircon Zr(SiO4) Titanite CaTiSiO5 Topaz Al2SiO4(F,OH)2 Image from mineral.galleries.com

  14. Oldest Material Recent evidence shows that the oldest materials on earth, Jack Hills Zircons, are 4.4 biliion years old. These record crystallization temperatures in the 600-750 ºC range - implying wet magmatic conditions and the possible establishment of the hydrosphere. NASA’s Earth Observatory

  15. Titanite and fluorite stability Testing titanite stability as a function of fluorine concentration in the melt Mount Scott Granite SQ-1 123a

  16. Titanite and Fluorite CaTiO5 (Ttn) + F2Û TiO2 (melt or ilm) + CaF2 (Fl) + SiO2 (melt or qtz) + ½ O2

  17. Titanite and Fluorite 8 The stability of these two phases in a natural granite, ASI near 1 ASI measures the ratio of Na, K, and Ca to Al F may easily complex with Al - could control its involvement in reactions 7 6 5 H2Om (wt.%) 4 3 2 1 0 0.0 0.5 1.0 1.5 2.0 Fm (wt.%)

  18. Sorosilicates Hemimorphite Zn4(Si2O7)(OH)2 H2O Epidote Ca2(Al,Fe)3(SiO4)(Si2O7)(O,OH) Image from mineral.galleries.com

  19. Image from Klein and Hurlbut, 1985

  20. Cyclosilicates Beryl - Be3Al2(Si6O18) Tourmaline - WX3Y6(BO3)3(Si6O18)(OH,F)4 Image from mineral.galleries.com

  21. Tourmaline Image from Balckburn and Dennen, 1988

  22. Pegmatites We’ve covered pegmatites before. But it’s worth reviewing because of it’s import to many ring silicates. A movie that illustrates the timing and processes of pegmatite (from Jill Banfield’s website, Cal-Berkeley)

  23. Ultramafic (very Mg-Fe rich) rocks Coarse: Dunite, Lherzolite, Harzburgite, Periodotite Fine: Kimberlite Minerals: these are rocks largely comprised of olivine (isolated silicate) and pyroxene (single-chain silicate) Origin: solidification of early Earth Location: the mantle Univ. North Carolina, Atlas of rocks, minerals, and textures website

  24. Obduction Henri de la Boisse

  25. Classification

  26. Kimberlite, lamprophyre Volcanic ultramafic rock. Very rare, and sourced straight from the mantle - eruption rates must be impressive. Some are diamondiferous. Univ. North Carolina, Atlas of rocks, minerals, and textures website

  27. Mafic (Mg-Fe rich) Rocks Coarse: Gabbro, Anorthosite Fine: Basalt Minerals: Largely olivine (isolated), pyroxene (single-chain), and feldspar (framework). Origin: Partial melting of the mantle Oceanic crust, oceanic islands, the moon. Anorthosite Univ. North Dakota, Plutonic Images Univ. North Carolina, Atlas of rocks, minerals, and textures website

  28. Basalt Gabbro Univ. North Carolina, Atlas of rocks, minerals, and textures website

  29. Beryl Image from Balckburn and Dennen, 1988

  30. Metamorphism Mineralogical and structural response of a rock to imposed conditions of T & P markedly different from those of its origin.

  31. Metamorphism • Could happen to any rock • Occurs in the solid state • Fluids may be present • A continuous process

  32. Closed system Isochemical The end product is a function of the starting material and the P-T history.

  33. Quartzite Conditions: low to moderate P & T Minerals: quartz (framework) Origin: sandstone

  34. Marble Conditions: low to moderate P & T Minerals: calcite or dolomite (carbonates) Origin: biogenic or chemogenitc (limestone or dolostone) Univ. of North Carolina Web atlas of metamorphic textures

  35. Silica Polymorphs

  36. More ‘morphs AlSiO5 CaCO3

  37. Open system Metasomatism The end product is is a function of the starting material, the P-T history, and fluid volume and composition.

  38. Some reactions Quartz-bearing Dolostone CaMg(CO3)2 + 2SiO2 = CaMgSi2O6 + 2CO2 Limestone + silica in fluid Ca(CO3)2 + 2SiO2(aq) = CaSiO3 + 2CO2

  39. Why the transformation? If you have a rock made of CaO, MgO, SiO2, CO2, the group of minerals that form at a given P&T is the lowest overall G of all possibilities. Each mineral is a phase. Each group of minerals is an assemblage.

  40. Metamorphic Facies All of these conditions are relevant to metamorphism on Earth. Note that some rocks will melt at lower T than others Grade - the degree of advancing metamorphic conditions P kbar Spear, 1993

  41. Heat and pressure

  42. Going down? Geothermal Gradient Burial Path

  43. Path back Prograde - the pathway to peak conditions. Retrograde - the path from peak conditions.

  44. Grid This can be mapped out using the thermodynamic boundaries for reactions. The presence or absence of minerals in rocks can illuminate the P-T pathway. Spear, 1993

  45. Spear, 1993 Reaction 8 Pg + Qtz = As + Ab+ H2O Pg = NaAl2(Si3)O10(OH)2 Paragonite Amphibole Reaction 11 Ms = Crn + Kfs + H2O Ms = KAl2(Si3Al)O10(OH)2

  46. Diagram from E.B Watson

  47. Slate Conditions: relatively low P & T Minerals: very fine grained feldspar (framework) and mica (sheet silicates) Origin: fine grained clastic (shale) Univ. of North Carolina Web atlas of metamorphic textures

  48. Schist Conditions: moderate P & T Minerals: fine grained micas (sheet), some feldspar or quartz (framework) may include garnet, staurolite (isolated) Origin: fine grained clastic (shale)

  49. Gneiss Conditions: high P & T Minerals: feldspar and quartz (framework), mica (sheet) Origin: clastic (shale and sandstone), felsic (rhyolite, granite) Univ. of North Carolina Web atlas of metamorphic textures

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