Day 27


These pages are under construction, and contian class notes....

NESOSILICATES

Olivine Group
Forsterite Mg2SiO4 Orthorhombic
Fayalite Fe2SiO4 Orthorhombic

Olivine Fo-Fa Transformation of Mg-rich olivine to spinel is thought to be responsible for increase in seismic velocities at top of earthÕs transition zone (e.g., p. 376). The total cation charge for oilvines and spinels is the same, and there is a solid solution series for spinel analogous to olivine. Olivine with spinel-structure has been foundin meteorites. Spinel is about 12% denser than olivine, for the same composition. Spinel MgAl2O4 Monoclinic Mg2SiIVO4 = Mg2SiIVO4 (b-spinel, see pg. 216-217).

Olivine has a distorted hexagonal closest-packed array of anions in which one eighth of the tetrahedral and one half of the octahedral interstices are occupied by cations; the spinel structure is the cubic closest packing analog of olivine.

forsterite+water=serpentine+brucite forsterite+quartz=enstatite dolomite+quartz=forsterite+calcite+CO2 fayalite is restricted to metamorphosed iron-rich lithologies

Garnet Group

Òpyralspite garnetsÓ
Pyrope Mg3Al2(SiO4)3 Isometric
Almandine Fe3Al2(SiO4) 3 Isometric
Spessartine Mn3Al2(SiO4)3 Isometric
Ò
ugrandite garnetsÓ
Grossular Ca3Al2(SiO4) 3 Isometric
Uvarovite Ca3Cr2(SiO4)3 Isometric
Andradite Ca3Fe2(SiO4)3 Isometric

General Formula
Structure and Symmetry
External growth forms
Cation substitutions
Garnet compositional changes as a function of P and T
Garnet occurrences in igneous, metamorphic, and sedimentary environments

Aluminum Silicates

Kyanite Al2SiO5 Triclinic
Sillimanite Al2SiO5 Orthorhombic
Andalusite Al2SiO5 Orthorhombic

Aluminum Silicate polymorphs
Phase diagrams
structural considerations of als occurrences
map of als occurrences in new england
tectonic reconstructions of new england
influence of heat flow and tectonic setting on als occurrence

Other Nesosilicates of Particular Importance

Chloritoid (Fe,Mg)2Al4O2(SiO4)2(OH)4 Monoclinic
- alternating brucite-like and corundum-like octahedral layers that are bridged by isolated SiO4 tetrahedra

Staurolite Fe2Al9O6(SiO4)4(O,OH)2 Monoclinic
- monoclinic, commonly as pseudo-orthorhombic, twinned crystals. Medium to high grade metamorphic rocks. - kyanite-like portions of unit cell

Titanite ('Sphene') CaTiSiO5 Monoclinic
- common Ti-bearing accessory. Monoclinic, commonly in wedge shapped crystals.

Topaz Al2SiO4(F,OH)2 Orthorhombic

Zircon ZrSiO4 Isometric


Syllabus...  Back...  Forward...