HEULANDITE, mineral of the zeolite group, consisting of hydrous calcium and aluminium silicate, Small amounts of sodium are commonly present, replacing part of the calcium. Crystals are monoclinic, and have a charac teristic coffin-shaped habit. They have a perfect cleavage par allel to the plane of symmetry on which the lustre is markedly pearly; on other faces the lustre is of the vitreous type. The mineral is usually colourless or white, sometimes brick-red. The hardness is 3 -4, and the specific gravity 2.2. It closely resem bles stilbite (q.v.) in appearance, and differs from it chemically only in containing rather less water of crystallization.
Heulandite occurs with stilbite and other zeolites in the amygdaloidal cavities of basaltic volcanic rocks, and occasionally in gneiss and metalliferous veins. The best specimens are from the basalts of Berufjord, in Iceland and the Faroe Islands, and the Deccan traps near Bombay. Crystals of a brick-red colour are from Campsie Fells in Stirlingshire and the Fassathal in the Tirol.