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: In thin section 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

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)
Calcite crystals in a marble. XPL image, 2x (Field of view = 7mm)	Calcite crystals in a marble. XPL image, 2x (Field of view = 7mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)
Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)
Twinning planes in calcite. PPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. PPL image, 2x (Field of view = 7mm)
Twinning planes in calcite. XPL image, 2x (Field of view = 7mm)	Twinning planes in calcite. XPL image, 2x (Field of view = 7mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)
Calcite crystals in a marble. XPL image, 10x (Field of view = 2mm)	Calcite crystals in a marble. XPL image, 10x (Field of view = 2mm)	Calcite crystals in a marble. XPL image, 10x (Field of view = 2mm)
Calcite crystals in a marble. XPL image, 10x (Field of view = 2mm)	Calcite crystals in a marble. XPL image, 10x (Field of view = 2mm)	Calcite crystals in a marble. XPL image, 10x (Field of view = 2mm)
Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)
Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)
Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. XPL image, 10x (Field of view = 2mm)	Twinning planes in calcite. PPL image, 10x (Field of view = 2mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Calcite - CaCO3

Calcite - CaCO₃