PREHNITE, a mineral consisting of calcium hydrogen orthosilicate, H 2 Ca2Al2(SiO 4 )j. It crystallizes in the hemimorphic class of the orthorhombic system, but the hemimorphic character is usually obscured by twinning. Crystals are generally platy in habit, but they rarely occur singly and distinctly shaped; almost invariably they are closely aggregated together to form barrel-shaped or globular groups with a crystalline surface. This form, together with the pale oil-green colour, gives the mineral a very characteristic appearance. It is translucent and has a vitreous lustre. The hardness is rather over 6 and the spec. grav. 2-80-2-95. Crystals are pyro-electric. Prehnite is sometimes classed with the zeolites, since it occurs under the same conditions as these minerals and often in association with them: the small amount of water (4-4%) is, however, expelled only at a red heat and is therefore not water of crystallization.
Prehnite occurs as a mineral of secondary origin in the amygdaloidal cavities of basic igneous rocks, such as basalt and diabase, and less often, in veins in granite and gneiss. Fine specimens are found with zeolites in the volcanic rocks of several places in the south of Scotland, e.g. Old Kilpatrick in Dumbartonshire, Bishopton in Renfrewshire, Campsie Hills in Stirlingshire and in the neighbourhood of Edinburgh; also at Paterson and Bergen Hill in New Jersey, and with native copper in the trap-rocks of the Lake Superior region. In the French (at Le Bourg d'Oisans) and Tyrolese Alps it occurs with axinite, epidote, felspar, etc., lining crevices in gneiss. Large masses have been found at Cradock in Cape Colony, from which locality it was brought in the 18th century by Colonel Prehn, the governor of the colony; hence the names " Cape chrysolite " and prehnite (of A. G. Werner, 1789). Prehnite is sometimes cut and polished for small ornaments; it then somewhat resembles chrysoprase in appearance.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)