Leucite

Leucite (Gr. leukos, 'white'), a rock-forming mineral which occurs in the form of icositetrahedra belonging to the cubical system. It has a hardness = 5.5 - 6, and a specific gravity = 2.45 - 2.50. The colour is white, ash-gray, or smoke-gray. It usually contains many inclusions, such as olivine, augite, and other minerals, together with glass enclosures, gas-bubbles, and occasionally fluid lacunæ. Unlike cubical minerals, it exhibits a certain degree of double refraction, believed to be due to conditions of unequal tension existing within the crystals. When exposed to a temperature of 500° C. the crystals become perfectly isotropic. Leucite occurs only in volcanic rocks, and those in which it occurs have a restricted distribution.