Silicates JD Price

Silicate Structure

(SiO2)

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+ ) Shortcuts to mineral formulae

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

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

Image from Klein and Hurlbut, 1985 Olivine structure

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

Image from Klein and Hurlbut, 1985 Habits

Image from Klein and Hurlbut, 1985 Garnet Structure

Other nesosilicates and subsaturates Zircon Zr(SiO 4 ) Titanite CaTiSiO 5 Topaz Al 2 SiO 4 (F,OH) 2 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 Image from mineral.galleries.com

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

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

CaTiO 5 (Ttn) + F 2  TiO 2 (melt or ilm) + CaF 2 (Fl) + SiO 2 (melt or qtz) + ½ O 2 Titanite and Fluorite

F m (wt.%) H 2 O m (wt.%) 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 Titanite and Fluorite

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

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

Image from Balckburn and Dennen, 1988 Tourmaline

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) Pegmatites

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 Ultramafic (very Mg-Fe rich) rocks

Henri de la Boisse Obduction

Classification

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

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. Univ. North Carolina, Atlas of rocks, minerals, and textures website Anorthosite Univ. North Dakota, Plutonic Images Mafic (Mg-Fe rich) Rocks

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

Image from Balckburn and Dennen, 1988 Beryl

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.

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

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

Silica Polymorphs

More ‘morphs CaCO 3 AlSiO 5

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

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 Some reactions

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. Why the transformation?

P kbar Spear, 1993 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 Metamorphic Facies

Heat and pressure

Geothermal Gradient Burial Path Going down?

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

Spear, 1993 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. Grid

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

Diagram from E.B Watson

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

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

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

Granite Gneiss Fabric

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