L6 - Orthosilicates and Chain Silicates

Orthosilicates

Orthosilicates feature isolated $Si$ tetrahedra alongside complementary polygons containing other cations, such as octahedra, cubes, and cuboctahedra.

Olivine

A key example is olivine, with the general formula $XYSiO_4$ .

Examples of Olivines:

• Olivine: $(Fe,Mg)_2SiO_4$

• Forsterite: $Mg_2SiO_4$ (one end-member of the olivine solid solution)

• Fayalite: $Fe_2SiO_4$ (the other end-member of the olivine solid solution)

• Monticellite: $CaMgSiO_4$

Olivine constitutes the majority of the Earth's upper mantle. A complete solid solution exists between forsterite and fayalite. However, there's an incomplete solid solution up to $Ca_2SiO_4$ because the $Ca^{2+}$ ion is too large to fit in both octahedral sites, preferring 8-fold cubic sites.

Garnet

Another key example is garnet, which has the general formula $A_3B_2(SiO_4)_3$, where:

• $A$: 8-fold coordination site with large divalent cations.

• $B$: 6-fold site occupied by smaller trivalent cations.

Pyralspite Garnets:

• Pyrope: $Mg_3Al_2(SiO_4)_3$ - found in high-pressure rocks and acts as a good barometer.

• Almandine: $Fe_3Al_2(SiO_4)_3$ - common in middle/high-grade metamorphic rocks.

• Spessartine: $Mn_3Al_2(SiO_4)_3$ - found in low-grade metamorphic rocks.

The pyralspite series (pyrope, almandine, spessartine) is useful for determining a rock's pressure history because the three minerals form at different pressures.

Ugrandite Garnets:

• Uvarovite: $Ca_3Cr_2(SiO_4)_3$

• Grossular: $Ca_3Al_2(SiO_4)_3$

• Andradite: $Ca_3Fe^{3+}_2(SiO_4)_3$

Aluminosilicates

Aluminosilicates $(Al_2SiO_5)$ are polymorphs of each other, meaning they share the same composition but have different crystal structures. The three minerals are:

• Kyanite: higher $P$, lower $T$

• Sillimanite: higher $T$

• Andalusite: lower $T$, lower $P$

The presence or absence of these minerals helps in determining the metamorphic history of a rock.

Zircon

A final orthosilicate is zircon $(ZrSiO_4)$. It acts as a repository for elements (U and Th but not Pb), making it easy to use in radiogenic dating.

Chain Silicates

There are two main types of chain silicates: single chain silicates (pyroxenes) and double chain silicates (amphiboles).

Pyroxenes

Pyroxenes are prevalent in mafic and ultramafic rocks, as well as high-grade metamorphic rocks. They have the general formula $XYZ_2O_6$, where:

• $X$: a large mono- or divalent ion

• $Y$: a smaller divalent or trivalent ion

• $Z$: a small ion $(Si^{4+}$ or $Al^{3+})$

Examples of Pyroxenes:

• Orthopyroxenes (<5% Ca):

  • Enstatite: $MgSiO_3$

  • Hypersthene: $(Mg, Fe)SiO_3$

  • Orthoferrosilite: $FeSiO_3$

• Clinopyroxenes (5-50% Ca):

  • Diopside: $CaMgSi_2O_6$

  • Hedenbergite: $CaFeSi_2O_6$

  • Augite: $(Ca,Na)(Mg,Fe,Al,Ti)Si_2O_6$

It is possible to have a monovalent and trivalent ion rather than two divalent ions; examples include jadeite, aegirine, and spodumene.

Several solid solutions exist within the pyroxenes, where different elements can substitute freely among the crystallographic sites.

Amphiboles

Amphiboles have the general formula $W_{0-1}X_2Y_5Z_8O_{22}(OH^-,F^-)_2$, where:

• $W$: a large monovalent ion

• $X$: a smaller mono- or divalent ion

• $Y$: a smaller di- or trivalent ion

• $Z$: a small ion $(Si^{4+}$ or $Al^{3+})$

Several solid solutions also exist among the amphiboles.