Willemite

 Willemite is a zinc silicate mineral (Zn₂SiO₄) and a minor ore of zinc. It is highly fluorescent (green) under shortwave ultraviolet light. It occurs in a variety of colors in daylight, in fibrous masses, solid brown masses (troostite), and apple-green gemmy masses.

Willemite
Willemite from Namibia
General
Category	Silicate mineral
Formula (repeating unit)	Zn2SiO4
Strunz classification	9.AA.05 (10 ed) 8/A.01-20 (8 ed)
Dana classification	51.1.1.2
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3
Identification
Color	Colorless to white, gray, black, flesh-red, burgundy-red, pink, brown, dark brown, mahogany-brown, honey-yellow, yellow, apple-green, blue, pastel green, light blue, azure-blue
Crystal habit	Fibrous, botryoidal to massive
Cleavage	{0001}, {1120} - imperfect
Fracture	Irregular to conchoidal
Mohs scale hardness	5.5
Luster	Vitreous to resinous
Diaphaneity	Transparent to opaque
Specific gravity	3.9 – 4.2
Optical properties	Uniaxial (+)
Refractive index	nω = 1.691 – 1.694 nε = 1.719 – 1.725
Birefringence	δ = 0.028
Other characteristics	Strongly fluorescent; may be phosphorescent
References	[1][2][3]

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^[4] (it is occasionally spelled villemite).^[5][6][7] The troostite variety is named after Dutch-American mineralogist Gerard Troost.^[8]

Formation and associated mineralsEdit

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.^[9] 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.^[10] 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 (zinc oxide) and franklinite (Fe,Mn,Zn)(Fe,Mn)₂O₄ (an iron rich zinc mineral occurring in sharp black isometric octahedral crystals and masses). Franklinite and zincite are not fluorescent.

UsesEdit

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.^[11][12]

Willemite variety troostite from New Jersey

Crystal structure of willemite

Photo comparisonEdit

In natural and in fluorescent light

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.