AUGITE, an important rock-forming mineral of the pyroxene (q.v.) group. The name is now applied to aluminous pyroxenes of the monoclinic series, which are dark-green, brown or black in colour. The habit of well-shaped crystals in lavas is simple and very characteristic, consisting of the forms a (zoo), b (oio), nt (iio), and s (III). Twins with the orthopinacoid (Ioo) as twin plane are common. Chemically, augite is an isomorphous mix ture in which the compounds diopside (q.v.) and hedenbergite preponderate, but containing also alumina and ferric oxide and variable amounts of the (Mg,Fe") (clinoenstatite) molecule. The small amount of sodium usually present in the mineral occurs in solid solution as the acmite-jadeite molecule. The interpretation of augite analyses is not simple, and it is at present not possible to state the manner in which the sesquioxides are disposed in the constitution of the mineral. According to G. Tschermak's view, these enter in the form of the hypothetical silicate sometimes known as Tschermak sili cate, but a study of superior analyses of augite shows that this view cannot be sustained. It is believed by some investigators that alumina and ferric oxide (above that needed for the acmite jadeite molecules) enter as such in solid solution with diopside and hedenbergite. Thus interpreted an analysis of an augite from Hawaii and 6.82% and 3.36% has the molecular composition as follows : 69.12 % ; I % ; (acmite), 5.08%; 1.9o%; FeSiO3, 0.40%; Common augite is usually non-pleochroic, but varieties containing significant proportions of soda (aegirine augite) or titania (titan-augite) are noticeably so, the former be ing characteristically green and the latter violet in thin section. Augite is a common mineral of igneous rocks of medium and low silicity such as porphyrites, gabbros and basalts while the varieties aegirine-augite and titan-augite are characteristic of alkaline ig neous rocks, nepheline- and leucite-bearing lavas and intrusions, monchiquites, limburgites, etc. (C. E. T.)