Prehnite

prehnite [′prā‚nīt]

(mineralogy)

Ca₂Al₂Si₃O₁₀(OH)₂A light-green to white mineral sorosilicate crystallizing in the orthorhombic system and generally found in reniform and stalactitic aggregates with crystalline surface; it has a vitreous luster, hardness is 6-6.5 on Mohs scale, and specific gravity is 2.8-2.9.

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Prehnite 

(named after the Dutch colonel Prehn, who discovered the mineral on the Cape of Good Hope), a sorosilicate mineral that has the chemical composition Ca₂ Al[AlSi₃O₁₀]·(OH)₂ with a trace of Fe replacing the Al. Prehnite crystallizes in the rhombic system. Clearly formed crystals usually have a prismatic, tabular habit and are rare. Prehnite is mainly found as nodular aggregates with radial-columnar structures. It is greenish yellow or gray. Its hardness on Mohs’ scale is 6.5 and its density is 2,800–2,950 kg/m³.

Prehnite is an ordinary hydrothermal mineral that occurs in the cavities of basic effusive rocks together with such minerals as zeolites, calcite, and epidote. It also forms in metamorphic rocks, for example, skarns, and during the decomposition of plagioclases in gabbro and diabases.