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Silicate minerals

The orthosilicates M II SiO (M = Be, Mg, Mn, Fe, Zn) and ZrSiO are neso -silicates. Be SiO (phenacite) is unusual as both Be II and Si IV occupy tetrahedral four-coordinated sites; the other divalent cations instead occupy six-coordinated octahedral sites and often isomorphously replace each other as in olivine, (Mg,Fe,Mn) SiO. Zircon, ZrSiO, demands eight-coordination of the Zr IV cations due to stoichiometry and because of their larger ionic radius (84 pm). Also significant are the garnets,, in which the divalent cations (e.g. Ca, Mg, Fe) are eight-coordinated and the trivalent ones are six-coordinated (e.g. Al, Cr, Fe). Regular coordination is not always present: for example, it is not found in Ca SiO, which mixes six- and eight-coordinate sites for Ca II. Soro -silicates, involving discrete double or triple tetrahedral units, are quite rare: metasilicates involving cyclic 2 n − units of corner-abutting tetrahedra forming a polygonal ring are also known.