Di'allage

Di'allage (Gr. diallagē, 'interchange;' so called from its changeable colour), a mineral nearly allied to Augite (q.v.), and by some regarded as a variety of it. Its chemical composition is essentially the same. It is seldom found perfectly crystallised, but usually massive, granular, or disseminated, and is characterised by its markedly laminated structure. It is brown, gray, or green in colour, but shows a kind of metallic lustre when broken across the cleavage. This is due to the presence of microscopic tubular inclusions or leaflets, which are disposed parallel to the cleavage-planes or laminae. Di'allage is a rock-forming mineral of some importance, occurring as a primary constituent of Gabbro (q.v.).