Smithsonite

smithsonite

willemite

sphalerite

tarbuttite

Images

Formula: Zn(CO3)
Carbonate, calcite group
Crystal System: Trigonal
Specific gravity: 4.42 to 4.44 measured, 4.43 calculated
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).
Smithsonite from San Antonio - Image

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).
Smithsonite from Tsumeb - Image

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 Hudgill Burn Mine, Alston Moor, Eden, Cumbria, England, UK, rare yellow-green botryoidal smithsonite has been found (AESS).
Smithsonite from the Hudgill Burn Mine - Image

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).
Smithsonite from the Nelly James Mine - Image

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

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).
Smithsonite from Cookes Peak - Image

At the Tintic Mining District, Utah, USA, smithsonite is generally abundant and single crystals are known from several mines, but they are small, only reaching a millimeter or two. It occurs much more commonly as coatings on fracture surfaces and as vein fillings, and some larger specimens of this type are known. A single specimen from an unknown mine, measuring 7 cm, is composed of a matrix of quartz with botryoidal coatings of pale blue smithsonite on fracture surfaces; many parts of this specimen are covered by druses of azurite crystals (MinRec 55.2.221).
Smithsonite from Tintic - Image

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

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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