Gabbro

Gabbro (Ital.), a rock consisting essentially of the two minerals plagioclase Felspar (q.v.) and Diallage (q.v.). It shows a thoroughly crystalline granitoid texture, with no trace of any base. The plagioclase is a basic variety—labradorite being commonest, but anorthite is also sometimes present in abundance. The diallage may usually be noted by the pearly or metallocoidal lustre on its cleavage-planes. It is usually either brownish or dirty green in colour. Olivine is also often met with as a constituent of gabbro, and some apatite is almost invariably present. In certain kinds of gabbro other varieties of pyroxene appear; and amongst other minerals which occasionally occur in gabbro may be mentioned hornblende, magnesia-mica, magnetite, ilmenite, quartz. The rock is of igneous origin, and occurs in association with the crystalline schists as large amorphous masses or bosses. Sometimes also it appears in the form of thick sheets and bosses associated with volcanic eruptive rocks.