Calcite
Definition and meaning of Calcite in chemistry.
Calcite is the most stable crystalline form of calcium carbonate, CaCO3. This common rock-forming mineral makes up limestone, marble, and chalk, and it crystallizes in the trigonal, or rhombohedral, crystal system.
In more detail
Calcite forms whenever calcium ions and carbonate ions combine in water, whether through slow geological precipitation over thousands of years, biological processes that build shells and coral skeletons, or quick laboratory synthesis. The mineral is known for its perfect rhombohedral cleavage, meaning it breaks cleanly along flat planes at consistent angles.
Clear calcite crystals also show strong double refraction, a property called birefringence, which splits a single ray of light into two separate images visible when you look through the crystal. Calcite is moderately soft, with a Mohs hardness of 3, so a steel knife can scratch it easily, though a fingernail cannot.
Dilute hydrochloric acid reacts vigorously with calcite, producing visible bubbles of carbon dioxide gas. Geologists commonly use this fizz test in the field to quickly identify carbonate rocks and distinguish calcite from harder, non-reactive minerals like quartz. Extremely clear calcite crystals, known as Iceland spar, were once prized for polarizing prisms in microscopes because their strong birefringence separates a light beam so cleanly into two distinct images.
Key facts
| Formula | CaCO3 |
|---|---|
| Crystal system | Trigonal (rhombohedral) |
| Mohs hardness | 3 |
| Field | Inorganic Chemistry |
| Key property | Strong double refraction (birefringence) |
Dropping a few drops of dilute hydrochloric acid onto a piece of limestone produces visible bubbling as carbon dioxide gas escapes. The reaction follows CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g), and geologists use this simple fizz test to identify carbonate minerals in rock samples. A similar, much slower version of this same reaction happens naturally underground, as mildly acidic rainwater slowly dissolves limestone bedrock over thousands of years and gradually carves out vast underground cave systems.
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 slowly 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.
What everyday rocks contain calcite?
Limestone, marble, and chalk are all made mostly of calcite. Marble forms when heat and pressure recrystallize limestone underground, while chalk forms from the accumulated shells of tiny marine organisms.
Sources & references
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