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