Hemimorphite

hemimorphite

rosasite

cerussite

smithsonite

Images

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
Common impurities: Cu,Fe
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.

Localities

At Mapimi, Mexico, fluorellestadite pseudomorphs after hemimorphite have been found (KL p222).

At the Ojuela Mine, Mapimí, Mapimí Municipality, Durango, Mexico, hemimorphite occurs with rosasite. (Mindat photos).

At Roughton Gill, Caldbeck Fells, Cumbria, England, UK, hemimorphite is relatively common as radiating sprays of millimetre-sized platy crystals, which are occasionally associated with chrysocolla, aurichalcite, rosasite and in one case mottramite (JRS 11.14).

At the Roughton Gill mine, At Roughton Gill, Caldbeck Fells, Cumbria, England, UK, fine specimens of blue botryoidal hemimorphite occur. Rosasite is a common associate, but the botryoidal forms are not usually coated with other minerals beyond a little pyromorphite (JRS 14.13-14).

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 Joplin, Missouri, USA, hemimorphite pseudomorphs after calcite have been found (KL p225).

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).

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