Silica Polymerization and the Silicates
Silicate Structures: Silicon and oxygen constitute more than 95% of the earths crust by volume.
92% of crustal minerals are silicates, by volume.
In all minerals of Earth's crustal and upper mantle environments, silica occurs in tetrahedral coordination with oxygen. With four oxygen at the apices of a tetrahedron, coordianted about the silicon, the charge between each silicon-oxygen is 1, and 1 remains with each oxygen for bonding. The (SiO4)-4 unit is a fundamental packing unit of the silicates.
The various ways that (SiO4)-4 tetrahedra link, or are polymerized, to other cations, or other (SiO4)-4 tetrahedra, defines the basic silicate types*.
| Nesosilicates ('orthosilicates') |
(SiO4)-4 | Isolated (SiO4)-4 tetrahedra | Garnet, Aluminum Silicates, Olivine, Staurolite, etc. |
| Sorosilicates | (Si2O7)-6 | Pairs of (SiO4)-4 tetrahedra | Epidote Group Minerals |
| Cyclosilicates | (Si6O18)-12 | Rings of (SiO4)-4 tetrahedra | Beryl, Tourmaline, Cordierite |
| Inosilicates | (Si2O6)-4/ (Si4O11)-6 |
Infinitely extending strips of (SiO4)-4 tetrahedra | Single Chain: Pyroxenes, Pyroxenoids; Double Chain: Amphiboles. |
| Phyllosilicates | (Si2O5)-5 | Infinitely extending sheets of (SiO4)-4 tetrahedra | The micas |
| Tectosilicates | (SiO2)0 | Three-dimensional frameworks of (SiO4)-4 tetrahedra | Silica Polymorphs, Feldspars, Feldspathoids, Scapolite, Zeolites |
*Deep mantle, meteorite impact, and other ultrahigh-pressure settings may result in a more densely-packed SiO2-structure, with silicon in octahedral (6-fold) co-ordiantion with oxygen. This is the structure basis for stishovite, which is isostructural with rutile (TiO2, with Ti in octahedral coordination).
Tectosilicates are the silicate class built from three-dimensional networks of corner-sharing SiO4 tetrahedra. About 65% of Earth's crust is composed of tectosilicates. This group of minerals includes the SiO2 polymorphs, feldspars, feldspathoids, scapolites, and zeolites.
The SiO2 Polymorphs
Considering minerals composed of only SiO2
(along with water, in the case of amorphous SiO2),
there are several possible symmetries and distinct structures possible.
The SiO2 polymorphs comprise an extremely
important group with distinctive pressures and temperatures of occurrence.
| Quartz | SiO2 | Hexagonal | Common in igneous, metamorphic, and sedimentary rocks |
| Tridymite | SiO2 | Hexagonal | A high-temperature, low pressure SiO2 polymorph; certain volcanic rocks. |
| Crystobalite | SiO2 | Isometric | A high-temperature, low pressure SiO2 polymorph; certain volcanic rocks. |
| Coesite | SiO2 | Monoclinic | A high pressure polymorph; deep crustal and mantle rocks |
| Stishovite | SiO2 | Tetragonal | A high pressure polymorph; meteorites and impact metamorphism |
| Opal | SiO2-nH2O | amorphous spheres in hcp |
Forms by replacement or direct precipitation in sedimentary and hydrothermal environments. |