GARNET

any member of a group of common silicate minerals that have similar crystal structures and chemical compositions. They may be colourless, black, and many shades of red and green. Garnet any member of a group of common silicate minerals that have similar crystal structures and chemical compositions. Garnets are most commonly found in metamorphic rocks, but also in certain types of igneous rocks, and, usually in minor amounts, in clastic sediments and sedimentary rocks. They may be colourless, black, and many shades of red and green. The garnet is the birthstone associated with the month of January. Use of garnets as gems began with the ancient Egyptians, if not earlier. During the latter part of the 19th century, garnet bracelets and brooches were particularly popular. Most familiar during the peak of popularity were varieties of pyrope, cut either with a convex polished surface (en cabochon) or with facets. Garnet varieties have become known by misleading names, frequently consisting of a locality with the name of another mineral variety, such as Uralian emerald or Cape ruby. The hardness of garnets and their sharp fracture make them suitable as abrasives. Higher-grade garnets are used in fine sanding and polishing of wood, leather, glass, metals, and plastics. Lower-grade garnets are used as sandblasting agents and in nonskid surface coatings. At Gore Mountain, N.Y., U.S., garnet crystals average 13 cm (5 inches) in size but reach a maximum of a metre or so. Consisting principally of almandine and pyrope, they are the hardest of the abrasive garnets (approximately 8 on the Mohs scale). Garnets have also been mined in Maine and Idaho. Outside the United States (the world's leading producer), notable quantities have been produced in Australia, China, and India. The general formula for the chemical composition of garnets is A3B2(SiO4)3, in which the A may be divalent ions of calcium, magnesium, iron, or manganese, and B may be trivalent ions of aluminum, iron, chromium, or more rarely manganese, titanium, vanadium, or zirconium. Garnets may contain other metallic atoms in minor amounts as well as nonmetallic constituents other than silicon. Most natural garnets are mixtures of two or more of the following pure species: pyrope Mg3Al2(SiO4)3 almandine Fe3Al2(SiO4)3 spessartine Mn3Al2(SiO4)3 uvarovite Ca3Cr2(SiO4)3 grossular Ca3Al2(SiO4)3 andradite Ca3Fe2(SiO4)3 The first three (pyralspite, from pyrope, almandine, and spessartine) form solid solutions with one another, as do the last three (ugrandite, from uvarovite, grossular, and andradite). Pyralspite and ugrandite do not mix. For detailed physical properties, see silicate mineral (table). For gem varieties, see also pyrope; almandine; spessartine; uvarovite; grossular; andradite; and hessonite. Additional reading W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (196263), with a 2nd ed. in progress (1978 ); Paul H. Ribbe (ed.), Orthosilicates, 2nd ed. (1982). R.V. Dietrich