Calcite
Definition and meaning of Calcite in chemistry.
Calcite is the most stable crystalline form of calcium carbonate (CaCO3), a common rock-forming mineral that makes up limestone, marble, and chalk. It crystallizes in the trigonal (rhombohedral) system and is the thermodynamically stable polymorph of CaCO3 at ordinary temperatures and pressures.
In more detail
Calcite forms whenever calcium ions and carbonate ions combine in aqueous solution, whether through slow geological precipitation, biomineralization in shells and coral skeletons, or laboratory synthesis. It is noted for perfect rhombohedral cleavage and strong double refraction (birefringence), which lets clear specimens split a light ray into two separate images. Because it is a moderately soft mineral (Mohs hardness 3), it scratches with a steel knife but not a fingernail, and it reacts vigorously with dilute hydrochloric acid, fizzing as it releases carbon dioxide gas.
Key facts
| Formula | CaCO3 |
|---|---|
| Crystal system | Trigonal (rhombohedral) |
| Mohs hardness | 3 |
| Field | Inorganic Chemistry |
Dropping a few drops of dilute HCl onto a piece of limestone (calcite) produces visible effervescence as CO2 gas is released: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g).
Frequently asked questions
How does calcite differ from aragonite?
Both are polymorphs of calcium carbonate with the same formula, CaCO3, but calcite has a trigonal crystal structure and is stable at surface conditions, while aragonite is orthorhombic and gradually converts to calcite over geological time.
Why does calcite fizz with acid?
Acid supplies H+ ions that react with the carbonate ion, converting it to unstable carbonic acid, which decomposes into water and carbon dioxide gas, seen as bubbling.