section_path: Classification/Silicates/Orthosilicates
section_name: Orthosilicates
paragraph_text: Orthosilicates
  
  Orthosilicates consist of isolated tetrahedra that are charge-balanced by other cations. Also termed nesosilicates, this type of silicate has a silicon:oxygen ratio of 1:4 (e.g. SiO). Typical orthosilicates tend to form blocky equant crystals, and are fairly hard. 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 SiO – are structurally composed of one tetrahedron, and one Al in octahedral coordination. The remaining Al 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) SiO 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 SiO. The garnet group has a general formula of X Y (SiO), 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 in the Y position: pyrope (Mg Al (SiO)), almandine (Fe Al (SiO)), and spessartine (Mn Al (SiO)). The ugrandite garnets have Ca in the X position: uvarovite (Ca Cr (SiO)), grossular (Ca Al (SiO)) and andradite (Ca Fe (SiO)). While there are two subgroups of garnet, solid solutions exist between all six end-members.
  
  Other orthosilicates include zircon, staurolite, and topaz. Zircon (ZrSiO) is useful in geochronology as U can substitute for Zr; 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 SiO (F, OH), often found in granitic pegmatites associated with tourmaline, is a common gemstone mineral.