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Formula: Zn2SiO4
Nesosilicate (insular SiO4 groups)
phenakite group
Crystal System: Trigonal
Specific gravity: 3.89 to 4.19 measured, 4.224 calculated
Hardness: 5½
Streak: white
Colour: colourless, white, grey, black, pink, red, yellow, green, blue, brown
Solubility: Moderately soluble in hydrochloric and sulphuric acids
Common impurities: Al, Fe, Mn, Pb, Mg, Ca
Environments:
Metamorphic environments
Hydrothermal environments
Willemite is a secondary mineral formed in the oxidation zone of
epithermal (low temperature) veins, especially in zinc deposits, or in metamorphosed zinc ore bodies
such as at the famous locality for this and other fluorescent minerals, Franklin, New Jersey.
Localities
At the Berg Aukas mine, Grootfontein, Otjozondjupa Region, Namibia, willemite is the most abundant
supergene mineral. The most commonly associated minerals are
smithsonite, cerussite and
descloizite. Very rare associations with
anglesite,
hemimorphite and hydrozincite
have been noted,
but these are primarily microscopic
(R&M 96.2.141-142).
The Nelly James Mine, Miller Canyon, Miller Peak, Cochise County, Arizona, USA, is a former small surface
lead, copper,
zinc, gold and
silver mine located at an altitude of 7250 feet. Mineralisation is a vein deposit
Mindat).
The mine is now famous for fluorescent minerals collected from the dumps, including
calcite (fluoresces red),
hydrozincite (sky blue),
powellite (creamy-yellow),
smithsonite (crimson red),
sphalerite (yellow-orange) and
willemite (green).
Willemite is one of the more common fluorescent minerals found here and is often associated with
calcite. Massive willemite can be white, tan or brown in daylight. A rare crystalline variety that is blue in
daylight has also been found at the mine. Under shortwave UV the willemite fluoresces a bright green rivaling that
of willemite found at the famous Sterling Hill or Franklin mines in New Jersey. Under longwave UV the willemite
fluoresces a weak green. Under medium range UV the willemite fluoresces a weak to medium green. Some, but not all,
willemite from the mine exhibits a long-duration sustained luminescence for up to 10 seconds after the shortwave
UV source is removed
(R&M 97.1.48-56).
At the Potter-Cramer mine, Vulture Mining District, Maricopa county, Arizona, USA, willemite is abundant
in the oxidised zone. It fluoresces creamy yellow-green to creamy whitish green under SWUV, and also exhibits greenish
phosphorescence
(R&M 96.1.37).
At the Mammoth-St Anthony mine, Pinal county, Arizona, USA, pseudomorphs
of willemite after
cerussite have been found
(R&M 94.2.169).
At the Kabwe mine, Central Province, Zambia, willemite occurs intergrown with
smithsonite and
cerussite. An early generation is formed by
hypogene replacement of
sphalerite, but a much later generation also crystallises at a
much lower temperature (R&M 94.2.144-145).
Alteration
In the oxidation zone of epithermal veins primary
sphalerite
alters to
secondary
hemimorphite,
smithsonite
and manganese-bearing willemite.
Similarly galena alters to
anglesite, cerussite and
manganese-bearing willemite.
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