Silicates JD Price Silicate Structure Silicate Structure (SiO2) - PowerPoint PPT Presentation

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+ ) Y tri- or quadrivalent in 6-fold coordination (Al 3+ , Fe 3+ , Ti 4+ ) Z tri- or quadrivalent in 4-fold coordination (Al 3+ , Fe 3+ , Si 4+ , Ti 4+ )

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)

Nesosilicates Olivine Forsterite Mg 2 SiO 4 Fayalite Fe 2 SiO 4 Image from mineral.galleries.com

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

Olivine structure Image from Klein and Hurlbut, 1985

Garnet X 3 Y 2 (ZO 4 ) 3 Spessartine Mn 3 Al 2 (SiO 4 ) 3 Almandine Fe 3 Al 2 (SiO 4 ) 3 Pyrope Mg 3 Al 2 (SiO 4 ) 3 Grossular Ca 3 Al 2 (SiO 4 ) 3 Uvarolite Ca 3 Cr 2 (SiO 4 ) 3 Andradite Ca 3 Fe 2 (SiO 4 ) 3 Image from mineral.galleries.com

Habits Image from Klein and Hurlbut, 1985

Garnet Structure Image from Klein and Hurlbut, 1985

Other nesosilicates and subsaturates Aluminosilicate Al 2 SiO 5 {AlAl(SiO 4 )O} Andalusite - Sillimanite - Kyanite Staurolite (Fe, Mg,Zn) 2 Al 9 [(Si,Al) 4 O 16 ]O 6 (OH) 2 Zircon Zr(SiO 4 ) Titanite CaTiSiO 5 Topaz Al 2 SiO 4 (F,OH) 2 Image from mineral.galleries.com

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

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

Titanite and Fluorite CaTiO 5 (Ttn) + F 2 � TiO 2 (melt or ilm) + CaF 2 (Fl) + SiO 2 (melt or qtz) + � O 2

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

Sorosilicates Hemimorphite Zn 4 (Si 2 O 7 )(OH) 2 H 2 O Epidote Ca 2 (Al,Fe) 3 (SiO 4 )(Si 2 O 7 )(O,OH) Image from mineral.galleries.com

Image from Klein and Hurlbut, 1985

Cyclosilicates Cyclosilicates Beryl - Be 3 Al 2 (Si 6 O 18 ) Beryl - Be 3 Al 2 (Si 6 O 18 ) Tourmaline - Tourmaline - WX 3 Y 6 (BO 3 ) 3 (Si 6 O 18 )(OH,F) 4 WX 3 Y 6 (BO 3 ) 3 (Si 6 O 18 )(OH,F) 4 Image from mineral.galleries.com

Tourmaline Image from Balckburn and Dennen, 1988

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)

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

Obduction Henri de la Boisse

Classification

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

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 Anorthosite Univ. North Dakota, Plutonic Images Univ. North Carolina, Atlas of rocks, minerals, and textures website

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

Beryl Image from Balckburn and Dennen, 1988

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

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

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

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

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

Silica Polymorphs

More ‘morphs AlSiO 5 CaCO 3

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

Some reactions Quartz-bearing Dolostone CaMg(CO 3 ) 2 + 2SiO 2 = CaMgSi 2 O 6 + 2CO 2 Limestone + silica in fluid Ca(CO 3 ) 2 + 2SiO 2 (aq) = CaSiO 3 + 2CO 2

Why the transformation? If you have a rock made of CaO, MgO, SiO 2 , CO 2 , 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 .

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

Heat and pressure

Going down? Geothermal Gradient Burial Path

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

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

Spear, 1993 Reaction 8 Pg + Qtz = As + Ab+ H 2 O Pg = NaAl 2 (Si 3 )O 10 (OH) 2 Paragonite Amphibole Reaction 11 Ms = Crn + Kfs + H 2 O Ms = KAl 2 (Si 3 Al)O 10 (OH) 2

Diagram from E.B Watson

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

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)

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

Fabric Gneiss Gneiss Granite Granite

Making granites? Metamorphic materials that exceed the solidus for the system start to melt Migmatite Photo by Mike Brown (UMD)