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Prehnite is a hydrated aluminium and calcium silicate. It is formed in the hydrothermal contexts of eruptive rocks. It is often associated to calcite, quartz and zeolites.
It generally crystallizes in a massive way, as spheres, the distinct crystals are rather rare. One often associates it to zeolites but it is actually a phyllosilicate, i.e., a silicate where the SiO4 tetrahedra are arranged as sheets (like the mica). Prehnite has a light green to yellow colour, it is more rarely gray or pink.
Some deposits : the Cape of Good Hope (Republic of South Africa), in Mali (spheres of 5 cm in diameter), the area of Poonah in India (epimorphosis after laumontite), in China, in Canada (Asbestos), in Namibia (Copper Valley), Austria, Germany (Harz Mountains) and Scotland.
In France, superb aggregates were discovered close in Combe de la Selle, close to Bourg d’Oisans (crystals rounded of 5 cm). Prehnite is used as fine stone in jewellery.
The place of conservation of the type of this species (from Cape of Good Hope, Republic of South Africa) is not known.
HISTORY : Species dedicated to the Dutch colonel Hendrik von Prehn (1733-1785) who discovered the mineral
Species first described in 1788 by Abraham Gottlob Werner (1749-1817), German geologist, Bergwerk Akademie, Freiberg, Germany
Type-locality : The Cap, Republic of South Africa
CHEMICAL FORMULA : Ca2 Al2 Si3 O10 (OH)2
CRYSTAL SYSTEM : Orthorhombic
COLOR : Yellow, gray, green pale, dark green, green, pink
DIAPHANIETY : Semi-transparent to translucent
LUSTER : Vitreous to pearly
STREAK : White
MORPHOLOGIE : Crystals are rare, spherical aggregates, compact
HARDNESS : 6,0-6,5
CHEMICAL CLASS: 2,90
DENSITY : VIII - Silicates
GROUP : Prehnite
STRUNZ CLASS BEFORE 2001 : 8/G.07-30
STRUNZ CLASS AFTER 2001 : 9.DP.20
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