Willemite


Willemite is a zinc silicate mineral and a minor ore of zinc. It is highly fluorescent under shortwave ultraviolet light.
It occurs in all different colors in daylight, in fibrous masses, solid brown masses, and apple-green gemmy masses.
It was discovered in 1829 in the Belgian Vieille-Montagne mine. Armand Lévy was shown samples by a student at the university where he was teaching. Lévy named it after William I of the Netherlands
.
The troostite variety is named after Dutch-American mineralogist Gerard Troost.

Formation and associated minerals

Willemite is usually formed as an alteration of previously existing sphalerite ore bodies, and is usually associated with limestone. It is also found in marble and may be the result of a metamorphism of earlier hemimorphite or smithsonite. Crystals have the form of hexagonal prisms terminated by rhombohedral planes: there are
distinct cleavages parallel to the prism-faces and to the base. Granular and cleavage masses are of more common occurrence. It occurs in many places, but is best known from Arizona and the zinc, iron, manganese deposits at Franklin and Sterling Hill Mines in New Jersey. It often occurs with red zincite and franklinite 2O4. Franklinite and zincite are not fluorescent.

Uses

Artificial willemite was used as the basis of first-generation fluorescent tube phosphors. Doped with manganese-II, it fluoresces with a broad white emission band. Some versions had some of the zinc replaced with beryllium. In the 1940s it was largely replaced by the second-generation halophosphors based on the fluorapatite structure. These, in turn have been replaced by the third-generation TriPhosphors.

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