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Abstract
The composition of igneous amphiboles is extremely sensitive to magma composition and the nature of the co-existing phases. The extent of solid solution in amphiboles may be depicted in the basalt tetrahedron. They are more silica-undersaturated than the liquid from which they have crystallized for liquids containing about 11% normative nepheline. For liquids containing more than 11% normative nepheline the reverse is true. The existence of this compositional divide may be demonstrated for both intrusive and extrusive suites. Amphiboles are generally nepheline-normative, except when crystallizing from granodioritic to granitic magmas. Fractionation factors between amphibole and magma composition for MgO and FeO appear to increase with decreasing temperature; those for TiO 2 and Na 2O decrease with increasing pressure. The CaO content of amphiboles appears to be insensitive to the CaO content of the magma.
Published Version
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