Willemite
Zinc Silicate, Zn2SiO4
Crystal System: Trigonal
Hardness: 5.5
Density: 3.9-4.2

Willemite is an uncommon mineral that occurs in the oxidized zone of zinc ore deposits as a secondary or mesogene mineral. It is also found in marbles where it may be a product of metamorphic recrystallization of smithsonite. Willemite is commonly associated with calcite, zincite, and franklinite. Economically important masses of willemite occur at the Franklin and Sterling Hill mines in New Jersey, which are the most famous localities for it. Willemite is named in honor of King William I of the Netherlands.

Willemite is most often found as compact, granular or massive aggregates. Well formed prismatic crystals are rare. Very rare idiomorphic crystals of willemite occur in Altenberg, Belgium. It may range in color from translucent to opaque and colorless to yellow-green, red, brown or black, with a resinous to vitreous luster.

While most willemite is not fluorescent, some varieties of flouresce with a bright green color when exposed to ultraviolet light. The very bright green fluorescence of some of the willemite from the Franklin and Sterling Hill mines makes specimens from those localities popular with fluorescent collectors. Some willemite is also known to be phosphorescent, that is, to continue to emit light after the ultraviolet source has been removed (glow in the dark). A variety which displays excellent fluorescent and phosphorescent properties occurs in Pinal County, AZ.

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bkeller@rockhounds.com 8/10/95