AMPHIBOLE

any of a group of common rock-forming silicate minerals. any of a group of common rock-forming silicate minerals. Amphiboles occur in most igneous rocks as minor and major constituents and form the major component in many metamorphic gneisses and schists. (See Table.) The amphiboles are inosilicates. They have two planes of well-developed prismatic cleavages that intersect at angles of about 124 and 56. Many amphiboles develop elongate crystals, some becoming needlelike and fibrous. Several of these fibrous formsriebeckite, cummingtonite, anthophyllite, and tremoliteare collectively called asbestos. In general, amphiboles crystallize in either the orthorhombic or monoclinic system. The amphiboles show considerable variation in composition. Besides silicon (Si) and oxygen (O), their components include sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), aluminum (Al), hydrogen (H), and fluorine (F). The general chemical formula of the amphibole group may be expressed as: (Na,Ca)2-3(Mg,Fe3+,Al)5(Si,Al)3O22(OH,O,F)2 or(Mg,Fe2+,Fe3+,Al)7(Si,Al)8O22(OH,O,F)3. The amphibole structure consists of double chains of silicon-oxygen tetrahedrons in which alternate tetrahedrons link two single chains by sharing an oxygen atom. The double-chain groups are bonded to parallel adjacent double chains by the Na, Ca and Mg, Fe2+, Fe3+, Al ions of the crystal. There are only small differences between the ionic sizes of Mg, Fe2+, Fe3+, and Al, so that the positions in the crystal that are occupied by Mg can also be occupied by these similarly sized ions (i.e., they can substitute for each other). The ionic sizes of Ca and Na are similar enough that they can substitute for each other. Moreover, the ionic size of Al is close enough to the size of Si that a limited Al substitution for Si occurs. Additional reading W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (196263), with a 2nd ed. in progress (1978 ); Manual of Mineralogy (after James D. Dana), 20th ed. by Cornelis Klein and Cornelius S. Hurlbut, Jr. (1985); J.J. Papike and M. Cameron, Crystal Chemistry of Silicate Minerals of Geophysical Interest, Reviews of Geophysics and Space Physics, 14(1):3780 (1976); B.E. Leake (comp.), Nomenclature of Amphiboles, American Mineralogist, 63(1112):102352 (1978); David R. Veblen (ed.), Amphiboles and Other Hydrous Pyriboles: Mineralogy (1981); David R. Veblen and Paul H. Ribbe (eds.), Amphiboles: Petrology and Experimental Phase Relations (1982). William B. Simmons