Willemite

nesosilicate

Franklin

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
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.

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 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.

Common impurities: Al, Fe, Mn, Pb, Mg, Ca

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