Nesosilicate (insular SiO4 groups) phenakite group
Specific gravity: 3.89 to 4.19
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
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.
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).
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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