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

Silicates

JD Price

Silicate Structure

Silicate Structure

(SiO2)

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+) 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 Mg2SiO4 Fayalite Fe2SiO4

Image from mineral.galleries.com

Image from Klein and Hurlbut, 1985

With continued crystallization, what happens to the composition

• f the olivine s.s.

and the melt?

Solid solution

Image from Klein and Hurlbut, 1985

Olivine structure

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

Image from Klein and Hurlbut, 1985

Habits

Image from Klein and Hurlbut, 1985

Garnet Structure

Other nesosilicates and subsaturates Zircon Zr(SiO4) Titanite CaTiSiO5 Topaz Al2SiO4(F,OH)2 Aluminosilicate Al2SiO5 {AlAl(SiO4)O} Andalusite - Sillimanite - Kyanite Staurolite (Fe, Mg,Zn)2Al9[(Si,Al)4O16]O6(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 600-750 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

CaTiO5 (Ttn) + F2 TiO2 (melt or ilm) + CaF2 (Fl) + SiO2 (melt or qtz) + O2

Titanite and Fluorite

Fm (wt.%) H

2

O

m

( w t . % )

0.0 0.5 1.0 1.5 2.0 1 2 3 4 5 6 7 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

Titanite and Fluorite

Sorosilicates Hemimorphite Zn4(Si2O7)(OH)2 H2O Epidote Ca2(Al,Fe)3(SiO4)(Si2O7)(O,OH)

Image from mineral.galleries.com

Image from Klein and Hurlbut, 1985

Cyclosilicates Cyclosilicates Beryl - Be Beryl - Be3

3Al

Al2

2(Si

(Si6

6O

O18

18)

) Tourmaline - Tourmaline - WX WX3

3Y

Y6

6(BO

(BO3

3)

)3

3(Si

(Si6

6O

O18

18)(OH,F)

)(OH,F)4

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 Anorthosite

• Univ. North Dakota, Plutonic Images

Mafic (Mg-Fe rich) Rocks

Basalt Basalt Gabbro 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

• f its origin.

Metamorphism

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

CaCO3 AlSiO5

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(CO3)2 + 2SiO2 = CaMgSi2O6 + 2CO2

Limestone + silica in fluid

Ca(CO3)2 + 2SiO2 (aq) = CaSiO3 + 2CO2 Some reactions

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. 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

• f 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

• ut 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 + H2O Ms = KAl2(Si3Al)O10(OH)2 Reaction 8 Pg + Qtz = As + Ab+ H2O Pg = NaAl2(Si3)O10(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 Granite Gneiss Gneiss

Fabric

Photo by Mike Brown (UMD)

Metamorphic materials that exceed the solidus for the system start to melt Migmatite

Making granites?