Aragonite

carbonate

limestone

Formula: Ca(CO3) carbonate
Specific gravity: 2.947
Hardness: 3½ to 4
Streak: White
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
Environments:

Sedimentary environments
Hot spring deposits
Hydrothermal environments

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 serpentinite, dunite and peridotite.
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 Tsumeb, Namibia, aragonite is common in the dolomite ore, sometimes with copper or cuprite inclusions (R&M 93.6.539-540).

At the Llynclys quarrt, near Oswestry, Shropshire, England, UK, crystals of aragonite occur on ferruginous dolomite (RES p293).

At the Kabwe mine, Central Province, Zambia, a specimen of aragonite crystals on vanadinite has been found (R&M 94.2.122).

Alteration

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).

Common impurities: Sr,Pb,Zn

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