Prehnite - Encyclopedia

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Orthorhombic
    Chemistry : Ca₂Al₂Si₃O₁₀(OH)₂
    Rarity : Quite common

Prehnite is an hydrothermal mineral of cavities in basic volcanic rocks where it is associated with zeolites, apophyllite and calcite. It is also a mineral of the Alpine fissures, more particularly in the Oisans (French Alps), and certain copper deposits in association with the native copper (Lake Superior). It is a mineral that owes its name to Colonel Hendrick von Prehn who discover near the Cape of Good Hope in South Africa in the XVIII^th century. Transparent to translucent, the prehnite is colorless to yellowish-white and often pale green. It is rare in prismatic or tabular crystals and commonly forms crystalline reniform to stalactitic associations, its most common facies being the grouping of flattened crystals forming half-spheres or helmets. Crystals are rare, they are sought after by collectors, it is also a mineral cut for jewelry, including cabochons or also used as an ornamental stone.

French Alpine fissures have given some of the most beautiful specimens in the world. La Combe de la Selle near Bourg d'Oisans (Isère) is famous for its lamellar crystals with rhombic contours, and its hemispherical aggregates "in helmets" (right picture) with sometimes a diameter of more than 5 cm and often associated with asbestos. Nearby, the Balme d'Auris also provided excellent crystals. It is also found in the Aure Valley in the Hautes-Pyrénées, Col d'Osquich in the Pyrénées-Atlantiques, and more anecdotally in many other small localities.

No fake known for this species.