Tracy, Robert J. Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.
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A group of monoclinic calcic pyroxenes which have the general chemical formula (Ca,Mg,Fe)(Mg,Fe)-Si2O6, in which calcium is the dominant cation in the first cation position. Monoclinic pyroxene with substantial iron or magnesium in place of calcium is called pigeonite, and has a different crystal structure from augite. Augite is generally considered to be a combination of the four end members, diopside (CaMgSi2O6), hedenbergite (CaFe2+Si2O6), enstatite (Mg2Si2O6), and ferrosilite (Fe22+Si2O6), but it almost always has substantial aluminum and minor to substantial amounts of sodium, ferric iron, chromium, and titanium. These minor constituents enter as solid solutions of augite toward acmite (NaFe3+Si2O6), jadeite (NaAlSi2O6), calcium Tschermak's molecule (CaAl2SiO6), ureyite (NaCrSi2O6), and CaTiAl2O6. The amount of substitution by any of these minor components depends upon the bulk composition of the rock as well as the conditions of formation. Four common varieties of augite are omphacite, which has about 50% substitution of jadeite; aegerine augite, which has substantial acmite substitution; fassaite, which has a considerable amount of calcium Tschermak's molecule in solution; and titanaugite, which is rich in titanium and has a characteristic purple color. See also: Diopside; Enstatite; Pigeonite
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