What Is Calcite?
QUICK ANSWER
Calcite is a common mineral composed of calcium carbonate (CaCO3). It's one of Earth's most abundant minerals, forming the bulk of limestone, marble, chalk, and travertine. Calcite has a hardness of 3 on the Mohs scale and dissolves in weak acids, which is why limestone caves form.
Calcite is one of Earth's most important minerals, forming massive rock layers, decorating caves, and serving as a key building block for countless living organisms. Despite being relatively soft, calcite is incredibly widespread and economically significant. Understanding calcite reveals chemistry that links volcanic activity, ocean life, cave formations, and everyday materials like concrete.
What is calcite made of?
Calcite is calcium carbonate, with the chemical formula CaCO3. The structure contains alternating layers of calcium ions and carbonate groups (one carbon atom bonded to three oxygen atoms). Calcite forms hexagonal crystals when it has space to grow freely, often producing distinctive scalenohedral or rhombohedral shapes. Pure calcite is colorless or white, but small amounts of impurities (iron, manganese, other elements) can produce yellow, brown, pink, red, blue, green, and other colors. The same chemical composition can also form aragonite, a different crystal structure with the same formula.
What are calcite's properties?
Calcite has several distinctive properties. Hardness is 3 on the Mohs scale (between fingernail at 2.5 and copper coin at 3.5). It can be scratched by a steel knife or even a copper coin. The cleavage is excellent in three directions producing rhombohedral fragments when broken. Calcite has strong birefringence (double refraction): looking through a clear calcite crystal at text produces a doubled image because light is split into two beams. The mineral effervesces (fizzes) when dilute acid is applied, releasing CO2 gas as the carbonate reacts. Specific gravity is about 2.7.
Where is calcite found?
Calcite is extraordinarily widespread. The most common occurrence is in sedimentary limestone, formed from accumulated marine organism shells (which were made of calcite or aragonite that converts to calcite). Limestone covers vast areas worldwide, especially regions like the Midwest United States, parts of Europe, and many island nations. Calcite also forms marble when limestone undergoes metamorphism, travertine when calcium-rich water deposits it on land, chalk from microscopic marine organisms, and the speleothems (stalactites, stalagmites) decorating caves. Pure calcite crystals occur in vein deposits and as fillings in geodes.
Why is calcite important?
Calcite has many important uses. Limestone (mostly calcite) is the primary ingredient in Portland cement, which makes concrete. Marble is used for sculptures, buildings, and tile. Crushed limestone is used in road construction, agriculture (to neutralize acidic soils), and as a flux in steelmaking. Calcite's birefringence made it important historically for optical instruments like polarizing prisms. Aquatic life depends on dissolved calcium carbonate to build shells and skeletons. Even ocean chemistry relies on calcite dissolution and precipitation, affecting carbon cycling and climate. Calcite is one of the most economically significant minerals globally.
Calcite is calcium carbonate (CaCO3), one of Earth's most abundant minerals. It has a hardness of 3 on the Mohs scale, excellent cleavage, and reacts with acids by fizzing. Calcite forms limestone, marble, chalk, travertine, and cave formations like stalactites. It has countless uses from cement production and construction to agriculture and steelmaking, making it one of the most economically important minerals on Earth.
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