Willemite

willemite

cerussite

smithsonite

hemimorphite

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Formula: Zn2SiO4
Nesosilicate (insular SiO4 groups) phenakite group
Specific gravity: 3.89 to 4.19
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).

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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