Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia



PREHNITE (Prehnit—Prehnite—ѕренит) (Werner, A.G. 1789), after Colonel Hendrick von Prehn (1733-1785), who discovered the mineral at the Cape of Good Hope, South Africa.

Composition & Properties. Silicate – Ca2Al[6][(OH)2|AlSi3O10], orthorhombic system. According the specifics of its structure, it takes an intermediate place between the subclasses of chain and sheet silicates. Hardness 6-6.5. Density 2.8-3.0. Glass luster to pearl one. Cleavage in one direction. Fragile. Prehnite is usually found in the form of sphaerolites of radial-rayed structure and grained masses. For its crystals barrel-like, short-columnar or table shapes are typical, as well as mosaic structure. Transparent to translucent. Color: mainly pale green, light yellow to brownish-yellow, more rarely reddish; the best color is supposed to be sea-green, white, colorless,. Its color depends on the content of iron – from white to yellowish with pearls play of color. Its massive green aggregates are called Burgenlanden jade. It can contain such inclusions as chlorite, sometimes native copper, in this case it displays aventurescence. Inclusions of byssolite produce the effect of cat’s-eye in cabochons of prehnite.

Deposits. It is formed as a result of hydrothermal transformation of basic rocks and fills their cracks, and in volcanic rocks – amygdales. In Russia, in the region of Northern Timan, gem-quality prehnite was found in basalts in the form of filled geodes, up to 50 cm. in cross-section. In the Middle Urals, in pegmatites by the village of Shaytanka, they discovered dark green kidney-shaped aggregates and complete masses of this mineral. In the South Urals, sphaerolites and veins of pale green prehnite were found at the Blagodat’ Mt. mine, near Verkhneural’sk. In the region of Noril’sk, at the Oktyabr’skoye deposit, they discovered translucent sphaerolites of prehnite up to 10 cm. in diameter. Findings of green sphaerolites of prehnite with inclusions of native copper are known in the Krasnoyarsk Region, in the Minusinsk Dist. In Khakassia, spherulites of salad-green prehnite were found at the Kopejvskoye deposit. In Kazakhstan, findings of prehnite are not rare at the Sokolovskiy and Sarbaiskiy mines, as well as at the Kurzhunkulskiy mine and Dzhetygara deposits. In Georgia, in the region of the Kazbek Mt., alpine veins contain small crystals and sphaerolites of prehnite. In France, gem-quality prehnite was found in the form of kidney-shaped aggregates up to 12 cm. in cross-section – in alpine veins of the Bourg d’Oisans deposit near Grenoble; and also in the Haute Pyrenees. From Italy ornamental prehnite comes to the market; it is from Vall d’Fassa, South Tyrol and Vall d’Ala, Piedmont. In Spain, it is found in pegmatites of La Cabrera, near Madrid. Findings of prehnite are known also in Germany – in Saxony; and in the north of Great Britain – in Renfrew Dist., Scotland.

In South Africa, prehnite is discovered at the Cradock deposit, Eastern Cape Prov., and at the Phalaborwa mine, Northern Prov. Prehnite was extracted in Namibia, at the deposits Tafelkop and Doros, the region of Swakopmund. In India, its findings are known in the Mahäräshtra State, in the region of Poona and Kandivli. There is information about ornamental prehnite at deposits of Pakistan. In Japan at the Tamagawa mine, Iwate Pref., light green variety of prehnite has the local name japonite. In Australia, yellowish-green prehnite is found in cavities among basalts near Wave Hill, the Northern Territories; and botryoidal masses up to 40 cm. at Prospect mine, near Sydney, New South Wales; as well as stalactites up to 9 cm. at the deposits Mallali and Garrawilla, near Gunnedah. It is represented with radial-rayed aggregates and crusts up to 10 cm. thick, which display the effect of cat’s-eye after processing.

In the U.S.A., in the Michigan State, they discovered rose prehnite, colored with inclusions of native copper, it was found on the Isle Royal, Superior Lake. Initially, they took it for thomsonite. They extracted there as an ornamental stone translucent spotty green fibrous prehnite, which displayed the effect of cat’s-eye in cabochon. It had the local name patricianite, or turtle back. Besides, an ornamental rock of stripped texture, colored different tones of green and composed with aggregate of prehnite, was called zonochlorite. In New Jersey, accumulations of grained prehnite in traps reached several tons. Prehnite, available for faceting, up to 20 cm. in cross-section, is extracted there at the deposits Paterson, Passaic Co., and Chimney Rock quarry, Sommerset Co. In Virginia the size of found sphaerolites of prehnite reached 60 cm. In Canada, in the North-Western Territories, there are numerous deposits of ornamental prehnite in the Ontario Prov., as well as in British Columbia. In the Québec Prov., at the Jeffrey mine, near Asbestos, they found colorless crystals of prehnite up to 12 cm. long.

Synonyms. Adelite (adilite, aedelite), after the discovery location near Ädel, Sweden | Bostrichites | Chiltonite | Cape chrysolite, mistake, after the discovery location in the Northern Cape Prov., South Africa. | Coupholite | Elelite | Cape emerald, from South Africa. | Burgenlanden jade, after the discovery location near Bernstein, Austria | Jacksonite, after Amer. chemist Ch.T. Jackson | Kapchrysolith, Germ. | Koupholite, from Gk. “koyphos” – light and “litos” – a stone, obs. | Star green stone | Triphanspat | Wollastonite, mist. | Cape zeolite, mistake, after the discovery location in the Northern Cape Prov., South Africa

Quality improvement. Gamma-irradiation uneven colored sphaerolites of prehnite from Northern Timan make them brown, and following annealing gives them even green hue. In this case they often display opalescence, which make stones more high-quality.

Treatment. Translucent, nice colored varieties of prehnite are cut in cabochon. From transparent crystals they make faceted stones up to 30 ct., which are of collection interest mainly. In the Museum of Calgary, Canada, there is a faceted yellow prehnite from Australia, 38.2 ct.

Synthesis. For jewelry purposes they manufacture synthetic prehnite – chrombonite. In the faceted form it is used for imitation of Colombian emerald, because of the similarity of their optical and physical properties.

Similarity. Prehnite looks like apatite, brazilianite, chrysolite, chrysoprase, hemimorphite, jadeite, perclase, serpentine, wavelite.