Smithsonite

carbonate

sphalerite

Formula: Zn(CO3) Carbonate
Specific gravity: 4.3 to 4.5
Hardness: 5
Streak: White
Colour: Colourless, white, yellow, brown, red, green, blue, grey
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Sedimentary environments
Hydrothermal environments

Smithsonite is one of the three main zinc supergene minerals, the others being hydrozincite and hemimorphite (JRS 18.14).
Smithsonite is often found as a secondary mineral in the oxidation zone of zinc ore deposits in limestone. It has also been observed in sedimentary deposits and as a direct oxidation product of sphalerite.
It is associated with sphalerite, galena, hematite, cerussite, calcite and limonite. It is often found as pseudomorphs after calcite. In the oxidation zone of epithermal veins sphalerite ZnS (primary) alters to secondary hemimorphite, smithsonite and manganese-bearing willemite.
It may form pseudomorphs after calcite (RES p148).

At Tsumeb, Namibia, smithsonite is associated with azurite and malachite (R&M 93.6.548).

At Cookes Peak mining district, Luna county, New Mexico, USA, smithsonite was the primary zinc ore, and is found in many places where heavy oxidation of sphalerite has occurred, usually on a limestone/gossan matrix (R&M 94.3.235-236).

At the Kabwe mine, Central Province, Zambia, smithsonite has been dound associated with tarbuttite or parahopeite or willemite (R&M 94.2.134-138).

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).

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). Smithsonite is found only in oxidised ore deposits (carbonate-rich), where hydrozincite is very rare, and hydrozincite, but not smithsonite, commonly occurs as coatings on mine walls and dumps, where the carbonate concentration is lower (JRS 18.14).

Common impurities: Fe,Co,Cu,Mn,Ca,Cd,Mg,In

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