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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, found in a vast array of geological settings. It is characterized by its perfect rhombohedral cleavage, relatively low hardness (3 on the Mohs scale), and effervescence when exposed to dilute acids. Calcite exhibits a wide range of crystal habits and colors, making it a visually diverse mineral. It is a fundamental component of many sedimentary rocks (limestone, chalk), metamorphic rocks (marble), and is a common vein mineral.
How to Identify
- Color
- Typically white or colorless, but can be tinted gray, yellow, green, blue, pink, brown, or black due to impurities.
- Luster
- Vitreous (glassy) to pearly on cleavage surfaces.
- Texture
- Can range from fine-grained to coarse-grained, massive, granular, fibrous, or stalactitic. Individual crystals are often smooth and well-formed.
- 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, hot spring deposits, caves (speleothems), and as a cementing agent in clastic sediments. It is also a major component of biological shells and skeletons.
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
- 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: Highly variable: rhombohedral, scalenohedral, prismatic, tabular, acicular, massive, granular, fibrous, stalactitic, oolitic, pisolitic, botryoidal.
- Cleavage Type: Perfect rhombohedral {1011}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to pearly
Formation
Calcite forms through a variety of geological 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, forming from the accumulation of biogenic debris or chemical precipitation. In metamorphic settings, it forms from the recrystallization of limestone into marble.
Usage
Calcite is a highly versatile mineral with numerous industrial applications. It is the primary component of limestone, which is used as a construction material (aggregate, cement production), agricultural lime (soil amendment), and flux in metallurgy. High-purity calcite is used in optical instruments (Iceland spar), as a filler in plastics, paints, and paper, and in pharmaceuticals and cosmetics. It is also used in environmental applications for acid neutralization.
Age Distribution
Ubiquitous throughout Earth's geological history, forming in various environments from Precambrian to present day.
Where to Find
Carlsbad Caverns, New Mexico, USA
Famous for spectacular speleothems (stalactites, stalagmites, flowstone) composed primarily of calcite.
Iceland
Historically known for 'Iceland Spar,' a transparent, high-purity calcite exhibiting strong birefringence.
Mexico
Various localities produce large, well-formed calcite crystals, often in diverse habits and colors.
Missouri, USA
Known for lead-zinc mining districts where calcite is a common gangue mineral, often forming large, clear crystals.
Worldwide Limestone and Marble Quarries
Calcite is the primary mineral in these rocks, found in vast quantities globally.
Finding Tips
Acid Test
Calcite effervesces vigorously (produces bubbles of CO2 gas) when a drop of dilute hydrochloric acid (HCl) is applied to its surface. This is a definitive test to distinguish it from similar minerals like dolomite (which reacts weakly or not at all unless powdered) or gypsum (which does not react).
Cleavage Recognition
Look for the characteristic perfect rhombohedral cleavage. Even in massive forms, small cleavage fragments will exhibit this shape. This helps differentiate it from quartz or feldspar.
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 helps distinguish it from harder minerals like quartz (7) or softer ones like talc (1).
Double Refraction (Birefringence)
Transparent calcite, especially in thicker pieces, will exhibit strong double refraction. If you place a clear piece of calcite over text, the text will appear doubled. This is a key optical property.
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
Scientific Classification
- Mineral Class
- Carbonates
- Group
- Calcite Group
- Crystal System
- Trigonal
- Chemical Formula
- CaCO3
- Composition
- Calcium Carbonate
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