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Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is one of the most common minerals on Earth, forming significant portions of the Earth's crust. It exhibits a wide range of crystal habits and colors, making it a fascinating and economically important mineral. 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, violet, brown, or even black due to impurities.
- Luster
- Vitreous (glassy) to pearly on cleavage surfaces.
- Texture
- Can be granular, massive, fibrous, stalactitic, or crystalline with various habits.
- Crystal Form
- Highly variable, including rhombohedral, scalenohedral, prismatic, tabular, and acicular forms. Twinning is common.
- Cleavage
- Perfect rhombohedral cleavage in three directions, producing characteristic rhomb-shaped fragments.
- Geological Environment
- Found in sedimentary rocks (limestone, chalk), metamorphic rocks (marble), hydrothermal veins, igneous rocks (carbonatites, amygdules), caves (speleothems), and as a cementing agent in clastic sediments.
Key Facts
- Hardness: 3 (Mohs scale)
- Specific Gravity: 2.71 g/cm³
- Crystal System: Trigonal
- Color: Colorless, white, gray, yellow, green, blue, pink, brown, black (due to impurities)
- Luster: Vitreous to pearly
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: Perfect rhombohedral in three directions at 74°55'
- Composition: Calcium Carbonate (CaCO3)
Quick Check
- Color: White, colorless, or various tints
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Extremely variable: rhombohedral, scalenohedral (dogtooth spar), prismatic, tabular, acicular, massive, granular, fibrous, stalactitic, oolitic, pisolitic, botryoidal, columnar, lamellar.
- Cleavage Type: Perfect rhombohedral {1011}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to pearly on cleavage surfaces, dull in massive forms
Formation
Calcite forms through various processes including precipitation from supersaturated aqueous solutions (e.g., in caves as stalactites and stalagmites, or in hot springs), biomineralization by organisms (e.g., shells of marine invertebrates, coral skeletons), and as a primary or secondary mineral in igneous, metamorphic, and sedimentary rocks. It is a major component of limestones and marbles.
Usage
Calcite is extensively used in construction (cement, aggregate), agriculture (soil conditioner, animal feed supplement), optics (Iceland spar for polarizing filters), pharmaceuticals (antacids, calcium supplements), and as a flux in metallurgy. It is also a primary source of lime (CaO) for various industrial applications.
Age Distribution
Found in rocks of all geological ages, from Precambrian to recent, as it forms in diverse environments.
Where to Find
Carlsbad Caverns, New Mexico, USA
Famous for spectacular speleothems (stalactites, stalagmites, flowstone) composed primarily of calcite.
Iceland
Historically known for large, clear crystals of 'Iceland spar' (optical calcite) used in polarizing optics.
Elmwood Mine, Tennessee, USA
Produces world-class, large, gemmy, golden-yellow calcite crystals, often associated with fluorite and sphalerite.
Mexico (various localities)
Known for diverse and aesthetic calcite specimens, including dogtooth spar and nailhead spar.
Worldwide in limestone and marble quarries
Calcite is the primary mineral in these rocks, found in massive and crystalline forms.
Finding Tips
Acid Test
Calcite effervesces vigorously (produces bubbles of CO2 gas) when a drop of dilute hydrochloric acid (HCl) is applied. This is a definitive test. Dolomite will only effervesce if powdered.
Cleavage Recognition
Look for the characteristic perfect rhombohedral cleavage, which produces fragments with parallel faces and angles of approximately 75° and 105°.
Hardness Test
Calcite has a Mohs hardness of 3, meaning it can be scratched by a copper coin (hardness ~3.5) but will scratch a fingernail (hardness ~2.5). This distinguishes it from harder minerals like quartz (hardness 7) and softer minerals like gypsum (hardness 2).
Double Refraction
Clear varieties of calcite (Iceland spar) exhibit strong double refraction, where a single line viewed through the crystal appears as two lines.
Similar Rocks
Aragonite
Aragonite (CaCO3)
Also known as: Calcium Carbonate
Dolomite
Dolomite (CaMg(CO3)2)
Also known as: Calcium Magnesium Carbonate
Gypsum
Gypsum (CaSO4·2H2O)
Also known as: Hydrated Calcium Sulfate
Quartz
Quartz (SiO2)
Also known as: Silicon Dioxide
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaCO3
- Composition
- Calcium Carbonate
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