Calcium carbonate

What is it?	Calcium carbonate is a common substance found in rocks, shells, and bones.
Where is it found?	It's in limestone, chalk, marble, and the shells of marine animals.
Uses	It is used in making chalk, antacids, and cement.
Fun fact	Calcium carbonate helps keep our bones strong!

Calcium carbonate is a chemical compound with the chemical formula CaCO₃. It is found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone. You can also find it in eggshells, gastropod shells, shellfish skeletons, and pearls. Materials that contain much calcium carbonate or look like it are called calcareous.

Crystal structure of calcite

Calcium carbonate is used in agricultural lime and can also be found as limescale, which forms when calcium ions in hard water react with carbonate ions. It has medical uses as a calcium supplement or an antacid.

Chemistry

Calcium carbonate is a type of carbonate. It reacts with acids to make carbonic acid, which turns into carbon dioxide and water. When heated, it releases carbon dioxide and forms calcium oxide, also called quicklime.

It can also react with water that has carbon dioxide, changing into soluble calcium bicarbonate. This process helps create caves and can lead to hard water in some areas. There is a special form of calcium carbonate named ikaite, which only stays stable in very cold temperatures below 8 °C.

Preparation

Most calcium carbonate used in factories comes from rocks. To make very pure calcium carbonate for food or medicine, people can use marble or make it from calcium oxide. They mix calcium oxide with water to get calcium hydroxide, and then they add carbon dioxide. This makes the calcium carbonate form, which is called precipitated calcium carbonate.

There is also a way to make calcium carbonate by mixing calcium chloride with ammonium carbonate in a special container called a desiccator. The ammonium carbonate breaks down into ammonia, carbon dioxide, and water. The carbon dioxide moves into the calcium chloride solution and helps create calcium carbonate.

Structure

Calcium carbonate, written as CaCO₃, is often found in nature as a mineral called calcite. Another form, called aragonite, can be made under special conditions, like higher temperatures. Scientists are still learning about a third form called vaterite. Other metals, like magnesium, can also form similar structures with carbonate.

Polymorphs

Calcium carbonate can form in three different ways, called polymorphs. The most common one is called calcite, followed by aragonite, and then vaterite, which is the least common.

These polymorphs have different shapes and structures. Calcite has a special shape called trigonal, aragonite has an orthorhombic shape, and vaterite can have more than one structure at the same time. All three can form together in water under normal conditions, but calcite is usually the main one that forms.

Crystal structure of calcite and aragonite

Some living things, like molluscs and arthropods, can make these different forms of calcium carbonate, often to build shells or for other protective purposes. They can choose which form to make, often using special molecules to help them do this.

Occurrence

Calcium carbonate is found in many places on Earth and beyond. It makes up important rocks like limestone, chalk, marble, and travertine. It is also the main part of eggshells, snail shells, and most seashells. Some green vegetables like broccoli and kale contain calcium carbonate too.

Calcite is the most stable polymorph of calcium carbonate. It is transparent to opaque. A transparent variety called Iceland spar (shown here) was used to create polarized light in the 19th century.

Scientists have found signs of calcium carbonate on Mars. This suggests there may have once been liquid water there, helping us learn more about the planet.

Geology

Calcium carbonate is found in many places in nature and stores a lot of carbon. It appears as minerals like aragonite, calcite, and dolomite, and it makes up rocks such as limestone, chalk, marble, travertine, and tufa.

Surface precipitation of CaCO3 as tufa in Rubaksa, Ethiopia

In warm, clear tropical waters, creatures like corals, plankton, algae, sponges, and mollusks help create calcium carbonate. These animals usually live in shallow waters where they can get sunlight and food. In colder waters, calcium carbonate can still form, but it grows very slowly. When ocean floor moves under continents, calcium carbonate can break down and release carbon dioxide.

Uses

Calcium carbonate is used in construction. It can be a building material or part of road mixtures. It is an ingredient in cement and is used to make lime for buildings. Because of damage from acid rain, it is not used alone for buildings anymore, but as a base for other materials.

500-milligram calcium supplements made from calcium carbonate

It helps clean iron in factories and keeps swimming pools balanced. In sugar factories, it removes unwanted materials. Chalk, made from calcium carbonate, was once used on blackboards, but most chalk today is made from a different material. Calcium carbonate is also used to help grow structures in water and as a filler in products like rubber gloves, diapers, and paper. It can make paints, plastics, and many everyday items cheaper and better. It is used in adhesives, tiles, and cleaning products.

Calcium carbonate is also used in health and food. It can add calcium to diets or help with stomach issues. It is found in medicines, toothpaste, and food like apple preserves. It helps balance the pH in soil and water to protect plants and fish. In factories, it is added to plastics to make them stronger.

Calcination equilibrium

Calcination of limestone using charcoal fires to make quicklime has been done for a very long time by many cultures around the world. Limestone turns into calcium oxide when heated to about 825 °C, but this can change with different conditions.

Calcium carbonate can change into calcium oxide and carbon dioxide depending on the temperature. At room temperature, calcium carbonate stays the same because the air has very little carbon dioxide. But when it gets hotter than 550 °C, calcium carbonate starts to release carbon dioxide. In a special kiln with charcoal fires, there can be more carbon dioxide than normal, so the process can happen at lower temperatures.

For this process to work well and quickly, the temperature needs to be around 898 °C.

Equilibrium pressure of CO₂ over CaCO₃ (P) versus temperature (T).
P (kPa)	0.055	0.13	0.31	1.80	5.9	9.3	14	24	34	51	72	80	91	101	179	901	3961
T (°C)	550	587	605	680	727	748	777	800	830	852	871	881	891	898	937	1082	1241

Solubility

Travertine calcium carbonate deposits from a hot spring

Calcium carbonate does not dissolve easily in plain water. It can mix better when the air has less of a certain gas, or when the water has special added ingredients.

When acids, like those in some cleaning products, are used, calcium carbonate can dissolve more. This helps remove hard spots in pipes or on surfaces caused by calcium carbonate.

CaCO₃ ⇌ Ca²⁺ + CO2−3

K_sp = 3.7×10⁻⁹ to 8.7×10⁻⁹ at 25 °C

HCO−3 ⇌ H⁺ + CO2−3

K_a2 = 5.61×10⁻¹¹ at 25 °C

H₂CO₃ ⇌ H⁺ + HCO−3

K_a1 = 2.5×10⁻⁴ at 25 °C

H₂O + CO₂(aq) ⇌ H₂CO₃

K_h = 1.70×10⁻³ at 25 °C

⁠P_CO₂/[CO₂]⁠ = H v {\displaystyle H_{\rm {v}}}

where H v {\displaystyle H_{\rm {v}}} = 29.76 atm/(mol/L) at 25 °C (Henry volatility), and P_CO₂ is the CO₂ partial pressure.

Calcium ion solubility as a function of CO₂ partial pressure at 25 °C (K_sp = 4.47×10⁻⁹)
P_CO₂ (atm)	pH	[Ca²⁺] (mol/L)
10⁻¹²	12.0	5.19×10⁻³
10⁻¹⁰	11.3	1.12×10⁻³
10⁻⁸	10.7	2.55×10⁻⁴
10⁻⁶	9.83	1.20×10⁻⁴
10⁻⁴	8.62	3.16×10⁻⁴
3.5×10⁻⁴	8.27	4.70×10⁻⁴
10⁻³	7.96	6.62×10⁻⁴
10⁻²	7.30	1.42×10⁻³
10⁻¹	6.63	3.05×10⁻³
1	5.96	6.58×10⁻³
10	5.30	1.42×10⁻²

H₂O ⇌ H⁺ + OH⁻

K = 10⁻¹⁴ at 25 °C

[A] (mol/L)	1	10⁻¹	10⁻²	10⁻³	10⁻⁴	10⁻⁵	10⁻⁶	10⁻⁷	10⁻¹⁰
Initial pH	0.00	1.00	2.00	3.00	4.00	5.00	6.00	6.79	7.00
Final pH	6.75	7.25	7.75	8.14	8.25	8.26	8.26	8.26	8.27
Dissolved CaCO₃ (g/L of acid)	50.0	5.00	0.514	0.0849	0.0504	0.0474	0.0471	0.0470	0.0470

[A] (mol/L)	[Ca²⁺] ≈ 0.5 [A⁻]	10⁻¹	10⁻²	10⁻³	10⁻⁴	10⁻⁵	10⁻⁶	10⁻⁷	10⁻¹⁰
Initial pH	2.38	2.88	3.39	3.91	4.47	5.15	6.02	6.79	7.00
Final pH	6.75	7.25	7.75	8.14	8.25	8.26	8.26	8.26	8.27
Dissolved CaCO₃ (g/L of acid)	49.5	4.99	0.513	0.0848	0.0504	0.0474	0.0471	0.0470	0.0470

[A] (mol/L)	1	10⁻¹	10⁻²	10⁻³	10⁻⁴	10⁻⁵	10⁻⁶	10⁻⁷	10⁻¹⁰
Initial pH	1.08	1.62	2.25	3.05	4.01	5.00	5.97	6.74	7.00
Final pH	6.71	7.17	7.63	8.06	8.24	8.26	8.26	8.26	8.27
Dissolved CaCO₃ (g/L of acid)	62.0	7.39	0.874	0.123	0.0536	0.0477	0.0471	0.0471	0.0470