Prehnite Prehnite Prehnite, Epidote General Category Silicate mineral Chemical formula Ca2Al(AlSi3O10)(OH)2 Identification Color Colorless to gray to yellow, yellow-green or white Crystal habit Globular, reniform to stalactitic Crystal system Orthorhombic - Pyramidal Cleavage Distinct on [001] Fracture Brittle Mohs Scalehardness 6 - 6.5 Luster Vitreous - pearly Specific gravity 2.8 - 2.95 Optical properties Biaxial (+) Refractive index nα = 1.611 - 1.632 nβ = 1.615 - 1.642 nγ = 1.632 - 1.665 Birefringence δ = 0.021 - 0.033 References [1][2][3] 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 theorthorhombic 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 blue 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 is an indicator mineral of the prehnite-pumpellyite metamorphic facies. It was first described in 1789 for an occurrence in Haslach, Harzburg and Oberstein, Germany, and named for Colonel Hendrik Von Prehn (1733-1785), an early Dutch governor of the Cape of Good Hope colony.[1]