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augite An important member of the pyroxene group of silicate minerals, occupying a field of composition within the calc-alkaline pyroxenes between the diopsides and the pigeonite field, and with the formula Ca(Mg,Fe)Si2O6 (Na may substitute for Ca and Al for Si; and a large increase in the amount of Na and Fe3+ (for Fe2+) gives aegirine augite (Na,Ca)(Fe3+,Mg,Fe2+)Si2O6, an important pyroxene in alkaline rocks); sp. gr. 3.3; hardness 6.0; monoclinic; greenish-black; crystals short, prismatic; a common constituent of basic igneous rocks but may also occur in high-grade metamorphic rocks. See also CLINOPYROXENE.