A rare mineral described in 1942 from volcanic rocks at Mafuru, in southwest Uganda. Kalsilite has since been synthesized. It is one of the three polymorphic forms of KAlSiO₄; the others are kaliophilite and an orthorhombic phase formed artificially at about 930°F (500°C). The mineral as shown by x-ray photographs is hexagonal. The specific gravity is 2.59. In index of refraction and general appearance in thin section it resembles nepheline and is difficult to distinguish from it. Structurally the two minerals are similar but belong to different crystal classes. The rock in which kalsilite was found has a dark-colored, fine-grained matrix with large olivine crystals and greenish-yellow patches. These patches are intimate mixtures of diopside, calcite, and kalsilite.