Willemite

Willemite

Willemite with franklinite from New Jersey
General
Category Silicate mineral
Formula
(repeating unit)		 Zn2SiO4
Strunz classification 9.AA.05
VIII/A.01-20 (8 ed)
Dana classification 51.1.1.2
Crystal system Trigonal
Identification
Color Colorless to white, gray, flesh-red, dark brown, honey-yellow, apple-green, blue
Crystal habit Fibrous, botryoidal to massive
Crystal symmetry Trigonal - rhombohedral
Point group 3
Space group R3
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]

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.[3]

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.[4] 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 These, in turn have been replaced by the third generation TriPhosphors.[5][6]

Willemite variety troostite from New Jersey
Crystal structure of willemite

See also

References

  1. ↑ http://rruff.geo.arizona.edu/doclib/hom/willemite.pdf Handbook of Mineralogy
  2. ↑ http://www.mindat.org/min-4292.html Mindat
  3. 1 2 http://webmineral.com/data/Willemite.shtml Webmineral
  4. ↑ Klein, Cornelis (2007). The Manual of Mineral Science, p.484. John Wiley and Sons, Inc., Hoboken. ISBN 9780471721574.
  5. ↑ Thayer, R. N. "The Fluorescent Lamp: Early U. S. Development".
  6. ↑ Kane, Raymond; Sell, Heinz (2001). "A Review of Early Inorganic Phosphors". Revolution in lamps: a chronicle of 50 years of progress. p. 98. ISBN 978-0-88173-378-5.

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