Willemite

Willemite is a zinc silicate (Zn2SiO4) crystallizing in the trigonal system. While often unremarkable in ordinary daylight, where it appears greenish-yellow, brown, or colorless, it is celebrated among collectors for its intense green fluorescence under shortwave ultraviolet light.

The mineral is a significant ore of zinc and is most famously associated with the zinc deposits at Franklin and Sterling Hill, New Jersey, which produced some of the world's most spectacular fluorescent mineral specimens.

A manganese-bearing pink variety is known as troostite. Willemite frequently occurs alongside franklinite and zincite in the unique Franklin assemblage.

Willemite forms primarily in metamorphosed zinc orebodies and in the oxidized zones of zinc deposits, where it develops as a secondary mineral from the alteration of sphalerite and other primary zinc ores.

The world-famous Franklin and Sterling Hill deposits in New Jersey formed through metamorphism of an ancient zinc-rich sedimentary protolith, creating a distinctive ore body where willemite, zincite, and franklinite occur together in calcite.

Willemite also occurs in carbonate-hosted ore deposits and in alkaline igneous rocks. Notable localities include Tsumeb (Namibia), Greenland, and various zinc districts worldwide.

In daylight, willemite is often nondescript, appearing greenish-yellow, honey-brown, reddish (troostite), or colorless, with a vitreous to resinous luster and a white streak. It has a hardness of about 5.5 and shows poor cleavage.

The definitive identification tool is shortwave ultraviolet light, under which willemite glows a vivid bright green, often with phosphorescence. This UV response distinguishes it from look-alikes such as olivine or epidote.

In the Franklin assemblage, look for willemite intergrown with black franklinite and red zincite in white calcite, a combination unique to that locality.

Willemite is an important ore of zinc and was historically mined extensively at Franklin and Sterling Hill. Its zinc content makes it economically valuable where it occurs in sufficient quantity.

The mineral has notable scientific and historical significance: the green-emitting phosphor zinc silicate doped with manganese was used in early fluorescent lamps, television tubes, and radar screens.

For collectors, fluorescent willemite specimens from Franklin are highly prized display pieces. The mineral is rarely faceted as a gemstone due to its softness, but transparent material occasionally yields collector gems.