AUGITE  , an important member of the

pyroxene (q.v.) group of rock-forming minerals . The name (from ai )al, 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 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 FIG . I . FIG . 2. 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 CaMg5i2O6, 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 .