PREHNITE  , a

mineral consisting of calcium hydrogen orthosilicate, H2Ca2Al2(SiO4)3 . 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. gray . 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 Renfrewshirc, 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, &c., lining crevices in gneiss . Large masses have been found at Credock 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 .