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Formula: Ca(CO3)
Carbonate, aragonite group, the high pressure orthorhombic
paramorph of trigonal
calcite; as such, it occurs in high pressure metamorphic rocks such as
those formed at subduction zones. A third paramorph is hexagonal
vaterite
(Wiki).
Varieties
Tarnowitzite is a lead-bearing variety of aragonite
Crystal System: Orthorhombic
Specific gravity: 2.947 measured, 2.944 calculated
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
Common impurities: Sr,Pb,Zn
Environments:
Sedimentary environments
Metamorphic environments
Hydrothermal environments
Cave deposits
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
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 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).
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).
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