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 XVIIIth 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.

Tabular prehnite crystals from Imilchil, Morocco
Prismatic prehnite crystals from Jeffrey Mine, Canada
40.20 ct prehnite cabochon from Kayes, Mali
12.75 ct faceted prehnite from Australia

Prehnite in the World

The Jeffrey Mine (Abestos, Quebec) is the site that has produced the finest prismatic crystals, greenish white, perfectly formed. The geodes of basalt contain many extraordinary crystals. The most famous come from the district of Paterson (New Jersey), and Centerville (Virginia), as well as the Deccan traps in India (Pune and Mumbai), where prehnite inlays cover geodes support crystals of apophyllite and okenite. We can also point out the Calvinia deposit (South Africa) where calcite crystals pseudomorph after prehnite can reach 50 cm, and Djebel Melh in southern Morocco which provided beautiful pale green crystallizations exceeding 4 cm. Finally, it is necessary to underline the very beautiful balls of prehnites associated with the epidote discovered in the region of Kayes, in the west of Mali (main photo) and which can form masses of several kilos...

Prehnite in France

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.

Twinning and special crystallizations

Twinning not observable.

Fakes and scams

No fake known for this species.

Hardness : 6 to 6.5
Density : 2.8 to 2.95
Fracture : Irregular
Streak : White

TP : Translucent to semi-transparent
IR : 1.611 to 1.665
Birefringence : 0.021 to 0.033
Optical character : Biaxial +
Pleochroism : Weak
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : None
Radioactivity : None


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