Carbonate, the high pressure paramorph of calcite; as such, it occurs in high pressure metamorphic rocks such as those formed at subduction zones (Wiki).
Tarnowitzite is a lead-bearing variety of aragonite
Specific gravity: 2.947
Hardness: 3½ to 4
Colour: colourless to white or grey, often stained blue, green, red or violet by impurities; colourless in transmitted light.
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Common impurities: Sr,Pb,Zn
Hot spring deposits
Aragonite is found in the oxidised zones of ore deposits and in evaporites, hot spring deposits and limestone caves.
It is also found in some metamorphic and igneous rocks.
It occurs with siderite in iron
deposits and with calcite,
dolomite and other magnesium minerals in altered
Aragonite is a common constituent of limestone.
It is a mineral of the blueschist facies where it may be associated with glaucophane.
At Abbotsford and Bundoora, inner Melbourne, Victoria, Australia, aragonite occurs in cavities in basalt associated with calcite (AJM 20.1.30).
At the Mumbai District, Maharashtra, India, some specimens of aragonite to 7 cm have been found, mostly as overgrowths on laumontite (Minrec 34.1.36).
At Tsumeb, Oshikoto region, Namibia, aragonite is common in the dolomite ore, sometimes with copper or cuprite inclusions (R&M 93.6.539-540).
At Berg Aukas, Grootfontein, Otjozondjupa Region, Namibia, aragonite was one of the last minerals to form. It is found associated with descloizite and in the willemite vugs with rare pyromorphite (R&M 96.2.122).
At Brandy Gill, Caldbeck Fells, Cumbria, England, UK, aragonite is common in small cavities in iron-rich dolomite (C&S).
At the Llynclys quarry, near Oswestry, Shropshire, England, UK, crystals of aragonite occur on iron-rich dolomite (RES p293).
At the Big Bend area, Brewster county, Texas, USA, chalcedony pseudomorphs after aragonite have been found (KL p256).
At the Kabwe mine, Central Province, Zambia, a specimen of aragonite crystals on vanadinite has been found (R&M 94.2.122).
Aragonite and calcite are both forms of calcium carbonate. Aragonite is less stable than calcite under atmospheric conditions, but the alteration of aragonite to calcite is a very slow reaction, so calcite and aragonite can co-exist in new rocks, but aragonite is a rare mineral in old rocks and shells. At extremely high pressure aragonite is the stable mineral (KB p71).
calcite and aragonite are precipitated according to the following reactions:
Carbon dioxide in the atmosphere is dissolved in rainwater forming weak carbonic acid:
H2O + CO2 → H2CO3
Carbonic acid dissolves limestone forming calcium bicarbonate
H2CO3 + CaCO3 → Ca(HCO3)2
This solution percolates into caves where calcium carbonate may be precipitated with the release of liquid water and gaseous carbon dioxide:
Ca(HCO3)2 ⇆ CaCO3 (solid) + H2O (liquid) + CO2 (gas) (R&M 91-4:329).
The net effect of these changes could be written as the reversible reaction
CaCO3 (solid) + H2CO3 (in solution) ⇌ Ca2+ + 2(HCO3)-
The forward reaction, solution of calcium carbonate, occurs in acid environments, and the reverse reaction, precipitation of calcium carbonate, occurs in strongly basic (alkaline) environments (KB p62).
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