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Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is extremely common and widely distributed throughout the Earth's crust. It is the principal constituent of limestone and marble, and a common mineral in hydrothermal veins, hot spring deposits, and as a cementing agent in sandstones. Calcite exhibits a remarkable diversity of crystal habits, making it a favorite among mineral collectors. Its defining characteristic is its perfect rhombohedral cleavage and its effervescence in dilute acids.
How to Identify
- Color
- Typically white or colorless, but can be tinted gray, red, orange, yellow, green, blue, brown, or black due to impurities (e.g., iron, manganese, magnesium, organic matter).
- Luster
- Vitreous (glassy) to sub-vitreous, sometimes pearly on cleavage surfaces.
- Texture
- Can be massive, granular, fibrous, stalactitic, botryoidal, or crystalline. Crystals can be blocky, tabular, prismatic, scalenohedral, or rhombohedral.
- Crystal Form
- Hexagonal-rhombohedral crystal system. Exhibits a vast array of crystal habits, including acute or obtuse rhombohedra, scalenohedra (dogtooth spar), tabular forms, and prismatic forms. Twinning is very common, especially contact twins.
- Cleavage
- Perfect rhombohedral cleavage in three directions, producing characteristic rhomb-shaped fragments. The cleavage angles are approximately 74° and 106°.
- Geological Environment
- Sedimentary rocks (limestone, chalk), metamorphic rocks (marble), hydrothermal veins, cave deposits (speleothems), hot spring deposits (travertine), and as a cementing agent in clastic sediments.
Key Facts
- Hardness: 3 (Mohs scale)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Hexagonal (trigonal division)
- Color: Colorless, white, gray, yellow, green, blue, pink, brown, black (due to impurities)
- Luster: Vitreous to sub-vitreous, sometimes pearly on cleavage surfaces
- Transparency: Transparent to translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral in three directions at 74° and 106°
- Composition: Calcium Carbonate (CaCO3)
Quick Check
- Color: White, colorless, or various tints
- Luster: Vitreous (glassy)
- Streak: White
Physical Characteristics
- Crystal Habit: Extremely variable: rhombohedral, scalenohedral (dogtooth spar), prismatic, tabular, massive, granular, fibrous, stalactitic, botryoidal, oolitic, pisolitic.
- Cleavage Type: Perfect rhombohedral {1011}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous (glassy) to sub-vitreous, sometimes pearly
Formation
Calcite forms in a wide variety of geological environments. It is a primary constituent of sedimentary rocks like limestone and chalk, formed from the accumulation of marine organism shells and skeletons. It also forms through chemical precipitation from supersaturated waters in caves (speleothems like stalactites and stalagmites), hot springs (travertine), and as a cement in clastic sedimentary rocks. In metamorphic settings, it is the main component of marble, formed from the recrystallization of limestone. Hydrothermal veins can also host calcite, often associated with metallic ore deposits. It can also form as a secondary mineral from the alteration of calcium-bearing silicates.
Usage
Calcite is one of the most widely used minerals. It is the primary component of limestone, which is used as a building material (dimension stone, crushed stone for aggregate), in cement production, and as a flux in metallurgy. Pure calcite (Iceland Spar) is used in optical instruments due to its strong birefringence. It is also used as an agricultural soil conditioner, a filler in plastics, paints, and paper, and as an antacid in pharmaceuticals. In the chemical industry, it's a source of calcium oxide.
Age Distribution
Found in rocks of all geological ages, from Precambrian to Cenozoic.
Where to Find
Worldwide
Calcite is one of the most common minerals on Earth and can be found in virtually every country. Significant deposits are found wherever limestone and marble occur.
Mexico
Known for producing large, well-formed scalenohedral (dogtooth spar) calcite crystals, particularly from Santa Eulalia, Chihuahua.
USA
Numerous localities, including the Tri-State Mining District (Missouri, Kansas, Oklahoma) for large crystals, and various cave systems for speleothems. Extensive limestone and marble quarries across many states.
Iceland
Historically famous for 'Iceland Spar,' a transparent, colorless variety of calcite exhibiting strong birefringence, from Eskifjörður.
China
Produces a wide range of calcite crystal forms and colors, often associated with other minerals.
Finding Tips
Acid Test
Calcite readily effervesces (fizzes) when a drop of dilute hydrochloric acid (HCl) is applied to its surface, releasing carbon dioxide gas. This is a definitive test to distinguish it from similar minerals like dolomite (which fizzes weakly or not at all unless powdered) or aragonite (which also fizzes but has different crystal habits).
Cleavage Recognition
Look for the characteristic perfect rhombohedral cleavage. Even in massive forms, small cleavage faces can often be observed. Breaking a piece of calcite will typically yield rhomb-shaped fragments.
Hardness Test
Calcite has a Mohs hardness of 3. It can be scratched by a copper coin (hardness ~3.5) but will scratch a fingernail (hardness ~2.5). This helps differentiate it from harder minerals.
Birefringence (Double Refraction)
Transparent calcite crystals, especially Iceland Spar, exhibit strong double refraction. If you place a clear calcite crystal over text, the text will appear doubled. This is a unique optical property.
Geological Context
Search in areas with limestone or marble outcrops, in caves, or in hydrothermal vein deposits. Calcite is often found in association with sulfide minerals (e.g., galena, sphalerite) in ore deposits.
Similar Rocks
Aragonite
Aragonite (CaCO3)
Also known as: Calcium Carbonate
Dolomite
Dolomite (CaMg(CO3)2)
Also known as: Calcium Magnesium Carbonate
Magnesite
Magnesite (MgCO3)
Also known as: Magnesium Carbonate
Limestone
Limestone (primarily Calcite)
Also known as: Calcium Carbonate Rock
Marble
Marble (primarily Calcite)
Also known as: Metamorphosed Limestone
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Hexagonal (Trigonal)
- Chemical Formula
- CaCO3
- Composition
- Calcium (Ca) 40.04%, Carbon (C) 12.00%, Oxygen (O) 47.96% (by weight)
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