Hemimorphite

sorosilicate

smithsonite

Formula: Zn4(Si2O7)(OH)2.H2O
Sorosilicate (Si2O7 groups)
Specific gravity: 3.475
Hardness: 4½ to 5
Streak: White
Colour: colourless, white, pale blue, pale green, gray, brown
Solubility: Slightly soluble in hydrochloric acid
Environments:

Hydrothermal environments

Hemimorphite is one of the three main supergene zinc minerals, the others being smithsonite and hydrozincite. Hemimorphite is a high temperature secondary mineral found in the oxidation portion of zinc deposits, associated with smithsonite, sphalerite, cerussite, anglesite and galena.

At the Golconda mine, Brassington, Derbyshire, England, UK, hemimorphite has been found on baryte (RES p107).

At the Millclose mine, Darley Dale, Derbyshire, England, UK, hemimorphite occurs on baryte and on calcite (RES p98).

At the Wapping mine, Matlock Bath, Derbyshire, England, UK, hemimorphite occurs on fluorite, sometimes with minor baryte or limonite (RES p72).

At the Kabwe mine, Central Province, Zambia, hemimorphite is common though often inconspicuous. It occurs mainly as intergrowths with cerussite and goethite, and it has been found associated with tarbuttite and smithsonite, and also with zincolibethenite on a limonite matrix (R&M 94.2.125-126).

Alteration

The first stage in the formation of zinc supergene minerals is the oxidation of sphalerite to zinc sulphate, which is very soluble and remains in solution as zinc and sulphate ions:
ZnS + 2O2 → Zn2+ + SO42-
Hemimorphite forms only at high pH (very alkaline conditions), when Zn2+ ions in solution react with a mobile source of silicate ions, such as silicic acid, normally derived from the weathering of silicate minerals. Hemimorphite is therefore most abundant in areas where such a source exists, such as the Askrigg block of the Northern Pennine Orefield, UK, where there are numerous beds of chert (JRS 18.14).

Common impurities: Cu,Fe

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