Nesosilicates

Nesosilicates, also known as orthosilicates, are a class of silicate minerals in which the silicate tetrahedra (SiO4) are isolated and do not share oxygen atoms with other tetrahedra. Each SiO4 unit is coordinated by metal cations that balance the charge, so the tetrahedra remain discrete rather than forming chains, sheets, or a three-dimensional framework. The minerals in this group are typically described by formulas that reflect isolated SiO4 units linked to various cations such as Mg2+, Fe2+, Ca2+, Al3+, and other elements.

Representative nesosilicates include several well-known mineral groups. The olivine family, with end-members forsterite (Mg2SiO4) and fayalite (Fe2SiO4), consists of magnesium- and iron-rich silicates common in mafic and ultramafic rocks. The garnet group has the general A3B2(SiO4)3 formula and includes end-members such as pyrope Mg3Al2Si3O12, almandine Fe3Al2Si3O12, grossular Ca3Al2Si3O12, and andradite Ca3Fe2Si3O12. Aluminosilicate nesosilicates such as kyanite, andalusite, and sillimanite share the same SiO4 isolation but differ in crystal structure and chemical composition (Al2SiO5 in all three polymorphs). Other important nesosilicates include zircon (ZrSiO4), titanite or sphene (CaTiSiO5), and topaz (Al2SiO4(F,OH)2), among others.

Nesosilicates are common in a wide range of geological settings, from mantle-derived rocks to continental crust. Olivine-bearing rocks, metamorphic assemblages containing garnet or aluminosilicates, and zircon-rich granitic suites illustrate the diversity and economic and geochronological significance of this mineral class.