1911 Encyclopædia Britannica/Stilbite

by Leonard James Spencer

STILBITE, a mineral of the zeolite group consisting of hydrated calcium aluminium silicate, CaAl₂(SiO₃)₆+6H₂O. Usually a small proportion of the calcium is replaced by sodium. Crystals are monoclinic, and are invariably twinned, giving rise to complex groups and characteristic sheaf-like aggregates. The colour is usually white, sometimes red, and on the perfect cleavage (parallel to the plane of symmetry) the lustre is markedly pearly; hence the name stilbite given by R. J. Haüy in 1796, from Gr. στίλβειν, to shine. After the separation of heulandite from this species in 1818, the name desmine (from δέσμη, a bundle) was proposed, and this name is now employed in Germany. The hardness is 31/2; and the specific gravity 2·2. Stilbite is a mineral of secondary origin, and occurs with other zeolites in the amygdaloidal cavities of basic volcanic rocks; it is sometimes found in granite and gneiss, and exceptionally in metalliferous veins. It is abundant in the volcanic rocks of Iceland, Faeroe Islands, Island of Skye, Bay of Fundy, in Nova Scotia and elsewhere. Beautiful, salmon-pink crystals occur with pale green apophyllite in the Deccan traps near Bombay and Poona; white sheaf-like groups encrust the calcite (Iceland-spar) of Berufjord near Djupivogr in Iceland; and crystals of a brick-red colour are found at Old Kilpatrick in Dumbartonshire.  (L. J. S.)