Formula: Zn(CO3) Carbonate
Crystal System: Trigonal
Specific gravity: 4.42 to 4.44 measured, 4.43 calculated
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
Smithsonite is one of the three main zinc
supergene minerals, the others being
hydrozincite and hemimorphite
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 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).
The Nelly James Mine, Miller Canyon, Miller Peak, Cochise County, Arizona, USA, is a former small surface lead, copper, zinc, gold and silver mine located at an altitude of 7250 feet. Mineralisation is a vein deposit Mindat). The mine is now famous for fluorescent minerals collected from the dumps, including calcite (fluoresces red), hydrozincite (sky blue), powellite (creamy-yellow), smithsonite (crimson red), sphalerite (yellow-orange) and willemite (green).
Smithsonite is usually grey, white or tan in daylight. It can be found massive or as colourless to tan botryoids. The honeycombed variety is also present. Under shortwave UV light the response can be a bright crimson-red, bright pink, pale blue or deep blue. Under longwave UV the response is a muted tan with medium brightness. The response under middle range UV is about the same as longwave with more of a pinkish tan colour. Some smithsonite specimens exhibit a very brief sustained luminescence upon removal of the shortwave UV source. However, the possibility exists that calcite is intergrown with this smithsonite and is the cause of this brief sustained luminescence (R&M 97.1.48-56).
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).
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-
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).
The mineral formulae are:
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