HEULANDITE  , a

mineral of the zeolite group, consisting of hydrous calcium and aluminium silicate, H4CaAl2(SiO3)s + 3H20 . 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 31-4, 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 dis- tinguished 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); independ- ently, in 1822, H . J . Brooke arrived at the same result, giving the name heulandite, after the mineral collector, HenryHeuland .

Heulandite occurs with stilbite and other

zeolites in the amygdaloidal cavities of basaltic volcanic rocks, and occasion-ally 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 H4(Sr, Ba, Ca)Al2(SiOa)s+3H20, 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 .