Editing Mineral (section)

====Orthosilicates====
[[File:Andradite-172390.jpg|right|thumb|Black andradite, an end-member of the orthosilicate garnet group.]]
Orthosilicates consist of isolated tetrahedra that are charge-balanced by other cations.<ref name="DG116-117">{{harvnb|Dyar|Gunter|2008}}, pp. 116–17</ref> Also termed nesosilicates, this type of silicate has a silicon:oxygen ratio of 1:4 (e.g. SiO<sub>4</sub>). Typical orthosilicates tend to form blocky equant crystals, and are fairly hard.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 573</ref> Several rock-forming minerals are part of this subclass, such as the aluminosilicates, the olivine group, and the garnet group.

The aluminosilicates –bkyanite, andalusite, and sillimanite, all Al<sub>2</sub>SiO<sub>5</sub> – are structurally composed of one [SiO<sub>4</sub>]<sup>4−</sup> tetrahedron, and one Al<sup>3+</sup> in octahedral coordination. The remaining Al<sup>3+</sup> can be in six-fold coordination (kyanite), five-fold (andalusite) or four-fold (sillimanite); which mineral forms in a given environment is depend on pressure and temperature conditions. In the olivine structure, the main olivine series of (Mg, Fe)<sub>2</sub>SiO<sub>4</sub> consist of magnesium-rich forsterite and iron-rich fayalite. Both iron and magnesium are in octahedral by oxygen. Other mineral species having this structure exist, such as [[tephroite]], Mn<sub>2</sub>SiO<sub>4</sub>.<ref>{{harvnb|Chesterman|Lowe|2008}}, pp. 574–75</ref> The garnet group has a general formula of X<sub>3</sub>Y<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>, where X is a large eight-fold coordinated cation, and Y is a smaller six-fold coordinated cation. There are six ideal endmembers of garnet, split into two group. The pyralspite garnets have Al<sup>3+</sup> in the Y position: [[pyrope]] (Mg<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>), [[almandine]] (Fe<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>), and [[spessartine]] (Mn<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>). The ugrandite garnets have Ca<sup>2+</sup> in the X position: [[uvarovite]] (Ca<sub>3</sub>Cr<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>), [[grossular]] (Ca<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>) and [[andradite]] (Ca<sub>3</sub>Fe<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>). While there are two subgroups of garnet, solid solutions exist between all six end-members.<ref name="DG116-117"/>

Other orthosilicates include [[zircon]], [[staurolite]], and [[topaz]]. Zircon (ZrSiO<sub>4</sub>) is useful in geochronology as U<sup>6+</sup> can substitute for Zr<sup>4+</sup>; furthermore, because of its very resistant structure, it is difficult to reset it as a chronometer. Staurolite is a common metamorphic intermediate-grade index mineral. It has a particularly complicated crystal structure that was only fully described in 1986. Topaz (Al<sub>2</sub>SiO<sub>4</sub>(F, OH)<sub>2</sub>, often found in granitic pegmatites associated with [[tourmaline]], is a common gemstone mineral.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 627–34</ref>