AUGITE

AUGITE (from the Greek word Auge, meaning shine, for the characteristic luster on the crystal faces and cleavage planes * EN: augite; DE: Augit; FR: augite; ES: augita; RU: авгит) is the rock-forming mineral, monoclinic pyroxene (clinopyroxene) (Ca, Mg, Fe2 +, Fe3 +, Al, Ti) 2 [(Si, Al) 2O6]. The composition is highly variable, Al2O3 (1-3%, less often 8-10%, up to 14% in the titanium augite), FeO (20%), Fe2O3 (up to 3%, less often 13%) and TiO2 (up to 3.0-5.7% in the titanium augite) are constantly present. Usual admixtures are Na, K, Mn, less often Ni, V, Cr.

Short prismatic crystals with the octagonal cross-section; the "swallowtail" twins are characteristic. It forms the phenocrysts (in the lavas), and the granular aggregates. The colour is black with the brownish or greenish tint, sometimes dark green. The hardness is 5-6. The density is 3400+100 kilograms per cubic metre. The cleavage is prismatic and average (with the angle of about 87 degrees between the planes); sometimes clear separateness (in the solid solution with diopside).

The zonal and sectorial structure of the grains is often observed in the thin-section metallographic specimens. The augite is of the magmatic origin; it is widespread mainly in the basic igneous rocks (gabbro, basaltes, dolerites), is less common in the middle (diorites, andesites, syenites) rocks, and ultramafic (ultrabasic) (picrites, peridotites, etc.) rocks, as well as in the rocks of the granulite and amphibolite facies.