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Geology is the Way

Calcite

Trigonal

CaCO3

Silicate minerals represent more than 90% of the Earth’s crust, producing the common rock-forming minerals of many igneous, sedimentary, and metamorphic rocks. The second most abundant class of minerals are carbonate minerals, which form about 4% of the crust. It might not sound like a lot, but carbonate minerals, like calcite and dolomite, are the building ‘bricks’ of the shells and skeletons of many organism and form an important group of rocks: carbonate rocks.

Calcite is the most abundant carbonate on Earth. It is one of the forms of calcium carbonate (CaCO3) and its name derives from the Latin word for lime, calx, and the related German word calcit. It was named as a mineral by Gaius Plinius Secundus (Pliny the elder) in 79 CE.

aragonite - calcite diagram

Pressure (P) – Temperature (T) diagram showing the stability fields of calcite and its high-pressure polymorph, aragonite. Modified after Prof. Stephen A. Nelson.

Structure and chemistry
Calcite is the most stable polymorph of CaCO3 on the Earth’s surface: the others are aragonite (stable at high pressure, metastable at ambient conditions) and the very rare and unstable vaterite. Calcite has a first-order structure that is similar to table salt (halite; NaCl), which has a regular, cubic arrangement of Na+ and Cl ions. In the structure of calcite, the positive ion is Ca2+, similar in ionic radius to Na+. However, the [CO3]2- ion is much larger than Cl and characterized by a triangular arrangement of oxygen atoms around a central carbon atom. Consequently, the ‘cube’ in calcite is distorted, producing a face-centered rhombohedral cell, with trigonal symmetry.
From a chemical point of view, calcite tends to be a nearly pure mineral, although some substitutions in the lattice may occur. CaCO3 has partial solid solutions with MgCO3 (magnesite), MnCO3 (rhodochrosite), and FeCO3 (siderite) and, indeed, Mg-,Mn-, and Fe-bearing calcite occurs in nature. Another element that may substitute Ca is Sr.

calcite crystal structure

The rhombohedral cell of calcite with its arrangement of calcium cations and bicarbonate anions. Graphics: Samuele Papeschi/GW.

calcite crystal habit

The trigonal symmetry of calcite allows various crystal habits. Top: rhombohedral habits, which can be flatter or more acute. Lower left: prismatic habit. Bottom center: scalenohedral habit, similar to a double bipyramid but formed by the combination of scalene triangles. Lower right: tabular habit. These are just examples, since more than 400 different crystal habits are known for calcite. Modified after Encyclopædia Britannica, 1911.


Calcite with rhombohedral habit complicated by oriented growth faces. Size: 5.6 x 5.4 x 3.8 cm. Wessels Mine, Northern Cape Province, South Africa. Photo by Robert M. Lavinsky.

Properties
Habit: rhombohedral, scalenohedral, prismatic, tabular, fibrous, acicular
Hardness: 3
Cleavage: {10-11} perfect rhombohedral cleavage
Twinning: {01-12} lamellar twinning: very common; {0001}: common; {10-11}: uncommon
Color: colorless to white, less commonly grey, yellow, green, blue, pink
Luster: vitreous, pearly
Streak: white
Alteration: dissolves in slightly acid waters
In thin section…
ε: 1.486-1.550
ω: 1.658-1.740
Color: colorless
Pleochroism: strong relief pleochroism
Birefringence (δ): 0.172-0.190 (fifth order colors)
Relief: high
Optic sign:
[Mindat]

calcite reaction with acid

The interaction between many carbonates and hydrochloric acid 10% produces a fizzy reaction. This is useful to identify calcite, a common constituent of carbonate rocks. Photo by Alessandro e Damiano.

Field features
Calcite is the main constituent of carbonate rocks and commonly occurs in the field as a component of limestones, marls, and fossils, but it can be found also in veins, as cement of other sedimentary rocks, as a mineral in igneous rocks, and in metamorphic rocks (e.g. marble). When found in its crystal form, calcite forms rhombohedra with rhombohedral cleavage and white to transparent color. Varieties of calcite can show colors ranging from pink to greenish, due to the presence of Mn, Fe, or Mg. In this case, they can be confused with magnesite or rhodochrosite, which have a similar habit. Calcite has also very low hardness (3) and hence can be scratched by glass or steel. The primary mean to recognize calcite in the field and to distinguish it from dolomite is the HCl test: calcite reacts with HCl (hydrochloric acid) diluted in a water solution at 10%, producing a fizzy reaction, whereas dolomite does not. Aragonite differs from calcite because it is orthorhombic, shows a prismatic habit, and lacks a rhombohedral cleavage (aragonite has a perfect prismatic cleavage).


Above: rhombohedral cleavage planes in a very large (width > 10 cm) calcite crystal in a vein. Moriglion di Penna, Lucca, Italy. [see post]


Above: cleavage planes and possibly twin planes in another very big calcite crystal (about 10 cm). Moriglion di Penna, Lucca, Italy. [see post]

euhedral calcite crystal

A euhedral calcite crystal showing well-developed rhombohedral habit crossed by rhombohedral cleavage planes. Moriglion di Penna, Lucca, Italy. [see post]

limestone

Limestone is a carbonate rock that largely consist of calcite. It is usually too fine-grained to recognize individual calcite grains but it reacts strongly with HCl 10%. Avane, Pisa, Italy.

Nummulitic limestone

Fossils in limestone, like the larger benthic foraminifera above, often consist of calcite that was secreted directly by organisms when they were alive or that substituted other minerals after burial. Width: 10 cm. Stone walls of Girona, Catalunya, Spain. [see post]

marble

Marble is a metamorphic rock that consist in large part of calcite crystals. Every glittering object in this rock is a tiny calcite crystal. Ortano Marble, Ortano, island of Elba, Italy.

concretions of calcite

Concretions of calcite are common in karst and hydrothermal environment. These are layered aggregates of calcite deposited in late veins associated with hydrothermal mineralizations. The bands with different color contain dolomite and various Fe minerals. Maffei Mine, Campiglia Marittima, Livorno, Italy.

Gallery – calcite crystals in the field (Monti Pisani, Italy) [blog post]

The birefringence of calcite
All minerals are birefringent, but the birefringence of calcite is so strong (>0.172) that it is already visible on hand samples. When light passes through calcite, it splits in two different rays, each with light waves vibrating perpendicular with respect to one another. One ray travels straight through the crystal (ordinary ray), the other is refracted in a misaligned fashion (extraordinary ray). When the rays emerge from the calcite crystal, they are separated. When looking through a large enough transparent calcite crystal, it is possible to observe this phenomenon, as images appear splat. When we rotate the calcite crystal, images will also rotate, as we change the orientation of the crystal (hence its optical axes) with respect to the entering light beam.

calcite spar iceland

Light is splat in two rays due to their strong birefringence of calcite. Photo by Pixabay.

Calcite in thin section
In thin section, calcite shows very high relief and appears colorless at PPL, displaying strong birefringence with characteristic very high five order colors at CPL. It has a perfect rhombohedral cleavage and very commonly shows lamellar twinning. Dolomite is identical to calcite in thin section and their distinction is very difficult [advice: check for dolomite in the hand sample before preparing the thin section]. The more straightforward way is to use alizarin red staining, which colors calcite in pink and leaves dolomite unstained. Another way to identify dolomite from calcite is to look at lamellar cleavage patterns in euhedral crystals. Both calcite and dolomite show lamellar twins parallel to the rhombohedral edges ({0001} twins), but calcite only shows twinning parallel to the long axis of the rhomb ({10-12}), whereas in dolomite twin planes occurs along both the long and short axis of the rhombs ({11-20} and {02-21}). However, these twins are enhanced by deformation (therefore more common in metamorphic rocks) and these observations require euhedral grains (very rare in calcite and dolomite aggregates).

calcite and dolomite in thin section

Twinning planes in calcite and dolomite. Modified after Tulane.edu (prof. Stephen A. Nelson).

CPL
CPL
CPL
PPL
PPL

Above: aggregate of twinned calcite crystals with lobate boundaries in a marble. CPL image. Width: 3.2 mm. Ortano Marble, Il Porticciolo, island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

Above
: large calcite crystal with evident twinning surrounded by smaller, recrystallized calcite grains in a marble. CPL image. Width: 4 mm. Monte Brugiana, Massa, Italy.

CPL
CPL
CPL
PPL
PPL

Above
: twinned calcite crystals in marble. CPL image. Width: 1.8 mm. Monte Brugiana, Massa, Italy.

Occurrence
Calcite is a primary mineral in many sedimentary environments. Various organisms use calcite to produce shells and skeletal parts. Hence, calcite forms fossils, bioclasts, and ooze in carbonatic environments. It also precipitates from water due to photosynthetic activity, evaporation, and other chemical processes, ending up in carbonate rocks and evaporites. Its ease of precipitation makes it a common cement also in other sedimentary rocks, like sandstones, conglomerates, and mudrocks. In igneous rocks, calcite may occur as a secondary mineral, for example as a precipitate in vesicles, and as a primary mineral in some silica-undersaturated rocks. Calcite is one of the primary constituents of carbonatites, rare rocks derived from the solidification of carbonatic magma or lava. Calcite is common also in metamorphic rocks, especially those derived from carbonate-rich metasediments like marble and calcschist. Finally, calcite occurs in veins and hydrothermal rocks, associated with a wide range of minerals.

References and Further Reading
Falini, G., Albeck, S., Weiner, S., & Addadi, L. (1996). Control of aragonite or calcite polymorphism by mollusk shell macromolecules. Science271(5245), 67-69.
Lorens, R. B. (1981). Sr, Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate. Geochimica et Cosmochimica Acta45(4), 553-561.
Teng, H. H., Dove, P. M., Orme, C. A., & De Yoreo, J. J. (1998). Thermodynamics of calcite growth: baseline for understanding biomineral formation. Science282(5389), 724-727.
Turner, F. J., Griggs, D. T., & Heard, H. (1954). Experimental deformation of calcite crystals. Geological Society of America Bulletin65(9), 883-934.
Wray, J. L., & Daniels, F. (1957). Precipitation of calcite and aragonite. Journal of the american chemical society79(9), 2031-2034.

        

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