Hydrozincite

minerals

smithsonite

hemimorphite

zinc

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Formula: Zn5(CO3)2(OH)6
Anhydrous carbonate containing hydroxyl
Specific gravity: 3.5 to 4.0
Hardness: 2 to 2½
Streak: White
Colour: White to grey, stained pale pink, or pale yellow or brown; colourless in transmitted light.
Solubility: Readily soluble in acids.
Common impurities: Fe,Cu
Environments

Hydrothermal environments

Hydrozincite is one of the three main zinc supergene minerals, the others being smithsonite and hemimorphite (JRS 18.14).

Hydrozincite is uncommonly formed in the oxidized portions of zinc-bearing deposits, generally as an alteration product of sphalerite, but also of hemimorphite and of smithsonite (Mindat, HOM). It also forms as incrustations in mine workings and caves (HOM).
Associated minerals include smithsonite, hemimorphite, willemite, cerussite, aurichalcite, calcite and limonite (HOM). Hydrozincite has been observed as pseudomorphs after dolomite (Mindat).

At the Kintore open cut, Broken Hill, New South Wales, Australia, hydrozincite occurs rarely, intergrown with aurichalcite (AJM 3.1.38).

At the Devon Cut deposit, Braeside lead field, Pilbara, Western Australia, hydrozincite is associated with hemimorphite, smithsonite and otavite (AJM 13.2.60).

At the Wheal Fortune copper mine, Northampton, Western Australia, hydrozincite forms as crusts associated with brochantite, serpierite and calcite (AJM 18.1.44)

At Narlarla, Western Kimberley district, Western Australia, hydrozincite occurs with cerussite, limonite and sphalerite (Dana).

The type locality is Bad Bleiberg, Gailtaler Alpen & Karnische Alpen, Carinthia, Austria.

At the Gowd Mine, Anarak District, Nain County, Isfahan, Iran, microcrystals of vanadium-bearing mimetite have been found on hydrozincite (R&M 91.5.406).

At the Roughton Gill mines, Caldbeck Fells, Allerdale, Cumbria, UK, encrustations of hydrozincite on sphalerite are rare, but do occur. They are probably the result of dump alteration (JRS 14.14).

At the Red Gill mine, Roughton Gill, Caldbeck Fells, Allerdale, Cumbria, England, UK, hydrozincite is extremely rare, but has been noted as an encrustation associated with hemimorphite (JRS 11.3.37).

At the Admiralty concession flat, Nentsberry Haggs mine, Alston Moor, Eden, Cumbria, UK, post-mining alteration has resulted in minor coatings of hydrozincite on sphalerite in some cavities (JRS 17.46).

At the Tynebottom mine, Alston Moor, Eden, Cumbria, England, UK, brianyoungite occurs in intimate association with crusts of hydrozincite (JRS 10.8).

At Grovebeck and Grinton mines, Northern Pennine orefield, UK, hydrozincite is a late-stage mineral filling cavities within hemimorphite; it is not interbanded with hemimorphite and no obviously post-mining coatings of the mineral have been observed (JRS 15.61).

At Whitaside mine, Northern Pennine orefield, UK, hydrozincite is interbanded with both hemimorphite and smithsonite. This relationship suggests that during supergene alteration here, the prevailing chemical and physical conditions locally fluctuated around the value of carbonate concentration that controls which of the carbonates, smithsonite (higher concentration) or hydrozincite (lower concentration), will form. No comparable interbanding of any of these species has been seen elsewhere within the orefield. No evidence has been seen at Whitaside of any hydrozincite of post-mining origin (JRS 15.60-61).

At the Cerro Gordo mine, Inyo county, California, USA, hydrozincite occurs with hemimorphite and willemite (Dana).

At the Galena District, Cherokee county, Kansas, USA, hydrozincite occurs with hemimorphite and smithsonite (Dana).

At the Kabwe mine, Central Province, Zambia, hydrozincite has been found on tarbuttite pseudomorphs after smithsonite (R&M 94.2.129-130)

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-
(JRS 18.14).

Where abundant oxygen is available, for example on vein exposures, within old mine workings, or in spoil heaps, low carbonate conditions are typical, resulting in the formation of hydrozincite as the normal zinc supergene species (JRS 15.60-61).

hydrozincite and CO2 to smithsonite and H2O
Zn5(CO3)2(OH)6 + 3CO2 ⇌ 5ZnCO3 + 3H2O
At pH between 5 and 8.5 (somewhat acid to somewhat alkaline) either hydrozincite or smithsonite will form, depending on the availability of carbonates. If this availability changes, then hydrozincite may change to smithsonite and vice versa, according to the above equation. Increased availability of carbonates favours the forward reaction and the formation of smithsonite. (JRS 15.60-61).

hydrozincite to zincite, CO2 and H2O
Zn5(CO3)2(OH)6 → 5ZnO + 2CO2 + 3H2O br H2O and CO2 are lost starting at about 230°, leaving zincite (Mindat).

The Activity-pH diagram below was calculated at 298.2 K for smithsonite, hydrozincite and adamite for constant activity (roughly equivalent to concentration) of H2AsO4- in solution, over a range of values of pH and of H2CO3 activity (MM 52.688).
stability smithsonite.jpg

The mineral formulae are:
smithsonite: Zn(CO3)
hydrozincite: Zn5(CO3)2(OH)6
adamite: Zn2(AsO4)(OH)














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