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Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia

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WILLEMITE


WILLEMITE (Willemit—Willémite—¬иллемит) (Lévy, A. 1830), after William I (1772-1843), the King of the Netherlands (1815-1840).

Composition & Properties. Silicate – Zn2[4][SiO4], subclass orthosilicates, trigonal system. Hardness 5.5. Density 3.8-4.1.Greasy luster to glass one. Perfect cleavage in two directions. Fragile. Transparent to non-transparent. Color: colorless, white, yellow, orange, yellowish-brownish, reddish-brown, green, rarely blue. It looks like hemimorphite. Willemite is a comparatively rare mineral, which is found in the form of short-columnar or needle crystals, radial-rayed and grained aggregates, as well as in wandering form. Manganese-containing variety of willemite is troostite of yellow, rose to red color, which is found in the form of columnar crystals up to 15 cm. long. The presence of hematite inclusions produces aventurescence in willemite, and sometimes even the weak effect of cat’s-eye.

Deposits. It is formed under contact metasomatism, in ore veins of zinc deposits, but mainly in the oxidation zones of zinc ores. In Russia, findings of willemite are know in East Transbaikalia, in the oxidation zone of the Kadainskiy mine; in Tyva Rep. – at the Sarygimateiskoye deposit, the Chikhachev Ridge; in the Primorskiy Region – at the Dal’negorskoye deposit. In Europe willemite, available for processing, was found in Belgium, at the Moresnet deposit, near the German border; in Germany – Altenberg, near Aachen; Czech Rep. – Vrançice. In Namibia, at the deposits Tsumeb and Guchab, they extract willemite in the form of honey-yellow crusts; also in Zambia – in the form of crystals at the Kabwe (form. Broken Hill) deposit. In Algeria willemite is found at the Bou Thaleb deposit; as well as in Morocco and Congo (Kinshasa, form. Zair). In China, in the Hunan Prov., at the Xingsao deposit, they found cobalt-containing willemite. In small volumes willemite is found at deposits of Greenland, Denmark. For the first time, willemite was discovered in the U.S.A., in New Jersey, at the Franklin zinc deposit, where they found its transparent green crystals up to 20 cm. and greenish-orange massive aggregates. There, they also found brown crystals of troostite up to 15 cm. long and its grained masses. In the same state, at the Sterling Hill mine, Sussex Co., willemite was discovered in the form of rose-brown crystals up to 2 cm. In the Canada, transparent bright blue crystals of willemite up to 3 cm. at the Mont St Hilaire, Québec, were found.

Synonyms. Belgite, obs. | Troostite, after the Amer. mineralogist G. Troost | Williamsite, obs.

Cut Gems. Transparent, usually yellow crystals are faceted; they receive gems not more than 10 ct. in mass. From the rough material of the Franklin deposit they made a faceted stone in 29.66 ct. Because of its low hardness, willemite is mainly of collection importance. Translucent material is cut in cabochons, including massive troostite, which is of special interest when there are inclusions of red zincite and white calcite in it. In the Smiths. Inst., Wash., there are two faceted yellow-orange willemites from the Franklin deposit, the U.S.A., in 11.1 and 11.7 ct.

Synthesis. Synthetic willemite is formed in products of metallurgical processing of zinc ores. It is synthesized as a strong luminophor.

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