1911 Encyclopædia Britannica/Augite
AUGITE, an important member of the pyroxene (q.v.) group of rock-forming minerals. The name (from αὐγή, lustre) has at various times been used in different senses; it is now applied to aluminous pyroxenes of the monoclinic series which are dark-greenish, brownish or black in colour. Like the other pyroxenes it is characterized crystallographically by its distinct cleavages parallel to the prism-faces (M), the angle between which is 87°. A typical crystal is represented in fig. 1, whilst fig. 2 shows a 
Fig. 1. Fig. 2.crystal twinned on the orthopinacoid (r ′). Such crystals, of short prismatic habit and black in colour, are common as phenocrysts in many basalts, and are hence known as “basaltic augite”; when the containing rock weathers to a clayey material the augite is left as black isolated crystals, and such specimens, usually from Bohemia, are represented in all mineral collections. Though typical of basaltic rocks, augite is also an important constituent of many other kinds of igneous rocks, and a rock composed almost wholly of augite is known as augitite. It also occurs in metamorphic rocks; for example, in the crystalline limestones of the Fassathal in Tirol, where the variety known as fassaite is found as pistachio-green crystals resembling epidote in appearance.
Chemically, augite resembles diopside in consisting mainly of CaMgSi2O6, but it contains in addition alumina and ferric iron as (Mg, Fe″) (Al, Fe‴)2 SiO6; the acmite (NaFe‴Si2O6,) and jadeite (NaAlSi2O6) molecules are also sometimes present. Variations in the amount of iron in mixtures of these isomorphous molecules are accompanied by variations in the optical characters of the augite. (L. J. S.)