Acmite is a member of the clinopyroxene group of inosilicates. Acmite is the sodium endmenber of the acmite-augite series. Acmite has the chemical formula NaFeSi2O6 in which the iron is present as Fe3+. In the acmite-augite series the sodium is variably replaced by calcium with iron(II) and magnesium replacing the iron(III) to balance the charge. Aluminium also substitutes for the iron(III).
Acmite occurs as dark green monoclinic prismatic crystals. It has a glassy luster and perfect cleavage. The Mohs hardness varies from 5 to 6 and the specific gravity is 3.2 to 3.4.
Acmite Commonly occurs in alkalic igneous rocks, also in regionally metamorphosed schists, gneisses, and iron formations; in blueschist facies rocks, and from sodium metasomatism in granulites. It may occur as an authigenic mineral in shales and marls.