CALCIUM CARBONATE: STRUCTURE, PROPERTIES, FORMATION, USES - CHEMISTRY - 2025

The calcium carbonate is an inorganic compound whose chemical formula is CaCO ₃. It is mainly found in minerals such as calcite and aragonite. It also makes up limestone, a sedimentary rock in which the mineral calcite is present.

This important metallic carbonate is obtained industrially by means of the extraction and milling of the minerals that contain it; marble is mainly used for this purpose. Another process lies in the use of calcium oxide, which is converted into calcium hydroxide, precipitating calcium carbonate from it by adding carbon dioxide. In this way crystals of a wide range of sizes are obtained.

The shell of snails is composed mainly of calcium carbonate. Source: Pixabay.

Clam shells, egg shells and oyster shells in which it is present can also be used for the industrial production of CaCO ₃ on a small scale.

The calcium carbonate present in limestone is dissolved by carbon dioxide in the water, generating calcium bicarbonate. This action can cause caverns and is a cause of alkalization of the water; event of great importance in the maintenance of life in it.

It has been used in the construction and elaboration of sculptures; examples of this are the Parthenon in Athens, the Cathedral of Krakow, and the sculpture of Abraham Lincoln in Washington. However, its susceptibility to acid rain has decreased its use in construction.

Calcium carbonate has had multiple applications in industry as a plastic and paper filler material. In medicine it has been used to control gastric acidity; as a dietary calcium supplement; to control phosphatemia in patients with chronic renal failure, etc.

Structure

CaCO3 crystal structure represented with a spatial filling model. Source: CCoil

The formula for calcium carbonate, CaCO ₃, indicates that the ratio of Ca ^{2+ to} CO ₃ ^2- ions is 1: 1; that is, for each Ca ²⁺ there is a counterpart CO ₃ ^2- interacting electrostatically with it. Thus, the ionic bond ends up arranging these ions to form structural patterns that define a crystal.

The upper image shows the structure of CaCO ₃. The green spheres correspond to the Ca ²⁺ cations, and the red and black spheres to the CO ₃ ^2- anions. Note that the structure appears to be made up of multiple layers: one of calcium and the other of carbonate; which means that it crystallizes into a compact hexagonal structure.

This hexagonal phase (β-CaCO ₃) corresponds to a polymorph. There are two others: orthorhombic (λ-CaCO ₃) and an even more dense hexagonal (μ-CaCO ₃). The image below helps to better visualize the happy hexagon:

Hexagonal structure of calcite. Materialscientist at English Wikipedia

However, depending on the temperature (and for this salt to a lesser degree the pressure) the ions in their vibrations readjust to other structures; these are the polymorphs already mentioned (β, λ and μ).

As they sound they are not very well known, unless they are mentioned with their mineralogical names in a future section.

Protein hardness

CaCO ₃ crystals are not alone: ​​they can host impurities such as other metal cations that color them; or proteins, incorporating an organic matrix that in some way contributes to its natural hardness.

Proteins reduce the tension that crystals experience against each other under pressure or impact. How? When sandwiched between the panes, acting as if it were a "pad" (similar to the brick-cement-brick set).

That is why this compound or mineral is biocompatible, and it is not surprising that it is part of the nails, seashells, shells or bones of hedgehogs. It has been a source of inspiration for those who are dedicated to developing new materials.

Properties

Other names

-Aragonite

-Calcite

-Volterito

-Calcium milk

-Whiteboard

-Marble

Molar mass

100.086 g / mol.

Physical description

Odorless white powder.

Taste

Chalk-like, tasteless.

Melting and boiling points

It decomposes because it releases CO ₂ before it even melts or boils.

Solubility

Practically insoluble in water and alcohol. It dissolves effervescently in dilute acetic and hydrochloric acids. Hydroxides however reduce its solubility. Meanwhile, ammonium salts and carbon dioxide increase the solubility of calcium carbonate in water.

Density

2.7 to 2.95 g / cm ³.

Decomposition

Around 825 ºC it decomposes into calcium oxide (lime) and carbon dioxide (CO ₂).

pH

8 to 9

Refractive index

-1.7216 at 300 nm and 1.6584 at 589 nm (calcite).

-1.5145 at 300 nm and 1.4864 at 589 nm (aragonite).

Incompatibilities

With acids, alum and ammonium salts.

Enthalpy of fusion

36 kJ / mol at 800 ° C (calcite).

Solubility Product Constant

3.36 · 10 ^-9 at 25 ° C.

Hardness

-3.0 (calcite)

-3.5 - 4.0 (aragonite) on the Mohs scale.

Phase transitions

Aragonite is metastable and irreversibly changes to calcite when heated in dry air at 400ºC.

Reactivity

Calcium carbonate reacts with acids, releasing carbon dioxide, ionic calcium, and water.

Calcium carbonate combines with water saturated with carbon dioxide, equivalent to carbonic acid (H ₂ CO ₃), to form calcium bicarbonate.

Training

Chalk, marble and limestone, the first two minerals and a rocky material the third, contain calcium carbonate and are of sedimentary origin. They are thought to have been formed by the sedimentation of snails over millions of years.

The pH could be the most important factor in the formation of polymorphs at a temperature of 24ºC. Vaterite is the main product in a pH range between 8.5 and 10; aragonite is at pH 11; and calcite at pH> 12.

Also, many freshwater algae are known to build calcite crystals when grown in a calcium-saturated environment. Furthermore, microalgae are capable of causing the precipitation of calcium carbonate.

Forms of calcium carbonate

The images below will show the three main forms or polymorphs for calcium carbonate:

Calcite crystal. Source: Parent Géry

Aragonite crystal. Source: Battistini Riccardo

Vaterite crystals. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0

From top to bottom are the polymorphs calcite, aragonite, and vaterite. Note at first glance the difference between the morphology of its crystals (the color is typical of its origins and characteristics of its surroundings).

Vaterite is more opaque than calcite, the latter even becoming completely transparent (Icelandic spar) and has therefore been used in jewelry and optical applications. Meanwhile, the aragonite crystals resemble small elongated monoliths.

If samples of these three polymorphs are observed under the electronic microscope, their crystals (with grayish tones due to the fact that the technique does not allow color resolution), the same morphologies would be found as in macroscopic scales; that is, with the naked eye.

Among these three polymorphs, calcite is the most abundant and stable, followed by aragonite, and lastly vaterite, the rarest of the forms of CaCO ₃

Applications

Industrial

Calcium carbonate increases the resistance to torsion and traction of synthetic and natural rubber, maintaining its flexibility. It is used in the construction industry as an ingredient in cement and as a raw material for lime. Its use has been decreased because it is damaged by acid rain.

Calcium carbonate is used in the purification of iron. In the form of lime, it removes the sulfur dioxide present in the metal. It is used in the purification of sugar from beets. It was once used as blackboard chalk, but has been replaced by plaster for this application.

The calcium carbonate is mixed with the putty that is used in the installation of glass. Ground is used as a filler material in the microporous film used in diapers. It is also used as a filler material in plastics such as PVC. In addition, it increases the strength of the plastic.

Calcium carbonate is used to increase the covering capacity of paints. It is used as a filling material for paper because it is cheaper than wood fiber, and can represent more than 10% of the paper.

Doctors

It is used as an antacid to combat gastric hyperacidity and relieve indigestion. It is used as a dietary calcium supplement and in the treatment and prevention of osteoporosis. It is used in the treatment of hyperphosphatemia in patients with chronic renal failure.

It has been used to reduce the undesirable effects of protease inhibitors used in the treatment of HIV, with a reduction in diarrhea being observed in patients.

It produces a decrease in blood pressure in pregnant women with hypertension and preeclampsia, since both could be associated with an increased demand for calcium due to the fetal presence.

Others

Calcium carbonate is used in agriculture as a fertilizer and to combat acidity in the soil. It is used as a preservative, retainer of color, and firming of food.

In addition, it is an ingredient in toothpaste, and serves as an abrasive in powder form in cleaning and washing.

References

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5. Greg Watry. (November 1, 2016). Discovering How Calcium Carbonate Crystals Get Strength. Advantage Business Marketing. Recovered from: rdmag.com
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