33 Photos
Willemite
Chemical Formula	Zn2SiO4
Species	Silicates
Crystal System	Trigonal
Mohs Scale	5
Specific Gravity	3.9-4.2
Color	Colorless to white, gray, flesh-red, dark brown, honey-yellow, apple-green, blue
Streak	White
Luster	Vitreous, Resinous
Refractive Index	n = 1.691 - 1.694 n = 1.719 - 1.725
Diaphaneity	Transparent, Translucent
Cleavage	Distinct/GoodGood to poor , poor
Fracture	Irregular/Uneven, Sub-Conchoidal
Crystal Habit：Fibrous, botryoidal to massive
Geological Setting：Occurs generally as a secondary zinc mineral derived from the supergene weathering of sphalerite (gossan & other oxidized deposits) or by the alteration of primary sphalerite in hydrothermal veins. The Franklin-Sterling Hill orebodies are unique in the context of massive willemite as a primary ore mineral, derived from an unknown proto-ore during Cambrian, or older, times.

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.

Formation and associated minerals

Willemite is usually formed as an alteration of previously existing sphalerite ore bodies, and is usually associated with limestone. 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.

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 These, in turn have been replaced by the third generation TriPhosphors.