Calcite - CaCO3

Named as a mineral by Gaius Plinius Secundus (Pliny the elder) in 79 from Calx, Latin for Lime, Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470 °C, and vaterite is even less stable.

The Carbonate, of which the best know is Calcite, contain a discrete (CO3)2- radical that may be considered as a single anion in the structure, but is in fact a trigonal planar complex. The triangular nature of the (CO3)2- radical dominates the structures of the carbonates and results in trigonal (rhombohedral) or orthorhombic (pseudo-hexagonal) symmetry.
The critical factor controlling the type of symmetry is the radius of the dominant metallic cation; for element such as Mn, Fe and Mg with a radius less than about 1.0A the carbonates are trigonal, buto for elements such as Ba, Sr and Pb with large radii the carbonates are orthorhombic.

Calcium lies close in radius value to the critical size, and this explains the existence of CaCO3 as two minerals, Calcite (trigonal) and Aragonite (orthorhombic). Although Aragonite is considered to be a high-pressure polymorph of CaCO3, it can grow at low pressures provided that the solution chemistry is correct. However, it is metastable and it usually invert to Calcite during recrystallization processes such as diagenesis.

Trigonal carbonate have a structure similar to halite, with alternating cations and anions in three dimension, trigonal carbonate are:

Calcite CaCO3
Magnesite MgCO3
Rhodochrosite MnCO3
Smithsonite ZnCO3
Cobaltocalcite CoCO3
Siderite FeCO3
Dolomite CaMg(CO3)2
Ankerite Ca(Mg,Fe)(CO3)2

Orthorhombic carbonate have large cations in a open hexagonal close pack alternating with planar carbonate ions lying in basal planes. Crystals are commonly pseudohexagonal. Orthorhombic carbonate are:

Strontianite SrCO3
Witherite BaCO3
Aragonite CaCO3
Cerussite PbCO3
Alstonite CaBa(CO3)2

Calcite is the low pressure polymorph and is the only truly stable form under surface condition. Calcite is almost pure CaCO3, although several cations may substitute Ca; calcite form a complete solidi solution with rhodocrosite, and partially with siderite, Smithsonite and magnesite.

Optical properties:
Color: Colourless
Form: Usually show fine to coarse anhedral aggregate
Cleavage: Perfect rhombohedral cleavage intersect at about 75°
Interference colors: V orders interference colors
Relief: Calcite is characterized by change in relief during a 360°rotation of the stage; the crystals is said to “twinkle” during rotation.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Calcite crystals in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. PPL image, 2x (Field of view = 7mm)
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Calcite crystals in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 2x (Field of view = 7mm)
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Twinning Planes in calcite crystals in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. PPL image, 10x (Field of view = 2mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)
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Twinning Planes in Calcite in a plutonic carbonatite (sovite) from Oka complex, Quebec, Canada. XPL image, 20x (Field of view = 1mm)