Willemite

Willemite
Willemite with franklinite from New Jersey
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, flesh-red, dark brown, honey-yellow, apple-green, 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

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 all different colors in daylight, in fibrous masses, solid brown masses ("troostite"), and apple green gemmy masses.

It was discovered in 1830 and named after William I of the Netherlands.

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. 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.

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 These, in turn have been replaced by the third generation TriPhosphors.