DIALLAGE, a variety of monoclinic pyroxene (q.v.) usually aluminous (see AUGITE), characterized by a prominent parting parallel to the orthopinacoid (Ioo) and often showing a fibrous structure. The schiller appearance of diallage is due as in bronzite (q.v.) to the interposition along the plane (I oo) of thin plates of ilmenite, which facilitate mechanical separation along this plane. Lamellar twinning on ( too) is also not uncommon. These in clusions probably result from unmixing in the solid state of ilmenite in solid solution in diallage at the temperature of crystal lization. Fine parallel intergrowths of diallage and rhombic pyroxene are not infrequent in gabbros, of which diallage is especially characteristic. It is scarcely represented among the younger volcanic rocks.
