Prehnite
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Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system. It is brittle with an uneven fracture and a vitreous to pearly lustre. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its color varies from light green to grey or white. It is translucent.
Though not a zeolite, it is found associated with them and with datolite, calcite, etc. in veins and cavities of basaltic rocks, sometimes in granites, syenites, or gneisses. It was first discovered in South Africa by Colonel Prehn, an early Dutch governor of the Cape of Good Hope colony.
Reference
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0471805807