Smithsonite

minerals

willemite

sphalerite

tarbuttite

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

Localities

At the San Antonio mine, Chihuahua, Mexico, smithsonite pseudomorphs after calcite have been found (KL p167).

At Tsumeb, Namibia, smithsonite is associated with azurite and malachite (R&M 93.6.548). Smithsonite pseudomorphs after aragonite and after azurite have been found here (KL p165, 166).

At the Berg Aukas Mine, Grootfontein, Otjozondjupa Region, Namibia, smithsonite directly replaced sphalerite, and, microscopically, willemite often replaced smithsonite. A typical paragenesis is sphalerite - smithsonite - willemite - ferric oxides. In the orebodies smithsonite was found either as granular masses forming a solid smithsonite rock, or as vein-filling botryoidal aggregates lining or filling fissures and cavities (R&M 96.2.133-136).

At the Philadelphia mine, Rush, Marion county, Arkansas, USA, smithsonite pseudomorphs after dolomite and after sphalerite have been found (KL p168-170).

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 found associated with tarbuttite, 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).

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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