A group of orthosilicate (= nesosilicate) minerals with stoichiometry of A3B2(SiO4)3, where divalent cations occur in the large-size (8-coordinated) A site and trivalent cations occur in the medium-size (6-coordinated octahedron) B site. Common garnet minerals are pyrope [Mg3Al2(SiO4)3], almandine [Fe3Al2(SiO4)3], spessartine [Mn3Al2(SiO4)3], uvarovite [Ca3Cr2(SiO4)3], grossular [Ca3Al2(SiO4)3], and andradite [Ca3Fe2(SiO4)3]. These garnets are separated into two series, pyralspite (the Mg, Fe, and Mn Al-rich members) and ugrandite (the Ca-rich members). Solid solution is common in the A site of pyralspite and in the B site of ugrandite, but little substitution between the two series. Grandite is a generic name for the grossular-andradite series. Garnet minerals commonly occur in metamorphic rocks, and are believed to be stable in the lower mantle of the earth. The minerals are generally cubic (= isometric) in symmetry. Garnet minerals are generally hard (Mohs hardness H: 6.5 to 7.5) and dense (specific gravity G: 3.5 to 4.5) and are used commercially as an abrasive.
Syn., silicate garnet