HEULANDITE, a mineral of the zeolite group, consisting of hydrous calcium and aluminium silicate, H-iCaA^SiOsJe + 3H ; O. Small amounts of sodium and potassium are usually present replacing part of the calcium. Crystals are monoclinic, and have a characteristic coffin-shaped habit. They have a perfect cleavage parallel to the plane of symmetry (M in the figure), 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, and varies from transparent to translucent. The hardness is 3^-4, and the specific gravity 2-2.
Heulandite closely resembles stilbite (q.v.) in appearance, and differs from it chemically only in containing rather less water of crystallization. The two minerals may, however, be readily distinguished by the fact that in heulandite the acute positive bisectrix of the optic axes emerges perpendicular to the cleavage. Heulandite was first separated from stilbite by A. Breithaupt in 1818, and named by him euzeolite (meaning beautiful zeolite); independently, in 1822, H. J. Brooke arrived at the same result, giving the name heulandite, after the mineral collector, Henry Heuland.
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, near Djupivogr, in Iceland and the Faroe Islands, and the Deccan traps of the Sahyadri mountains near Bombay. Crystals of a brick-red colour are from Campsie Fells in Stirlingshire and the Fassathal in Tirol. A variety known as beaumontite occurs as small yellow crystals on syenitic schist near Baltimore in Maryland.
Isomorphous with heulandite is the strontium and barium zeolite brewsterite, named after Sir David Brewster. The greyish monoclinic crystals have the composition H 4 (Sr, Ba, Ca)Al 2 (Si0 3 )6-l-3H2O, and are found in the basalt of the Giant's Causeway in Co. Antrim, and with harmotome in the lead mines at Strontian in Argyllshire. (L. J. S.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)