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Carbonate

Calcite

CaCO₃

About Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is one of the most common minerals on Earth, constituting a significant portion of the Earth's crust. It is the primary mineral in limestone and marble. Calcite is famous for its wide variety of crystal habits and its distinct physical properties, such as perfect cleavage and double refraction.

Formation & Geology

Calcite forms in almost all geological environments. It is a major constituent of sedimentary rocks (limestone), metamorphic rocks (marble), and igneous rocks (carbonatites). It also forms in hydrothermal veins and in caves as stalactites and stalagmites. It is the primary mineral used by marine organisms to build shells and coral skeletons.

Physical Characteristics

Calcite defines hardness 3 on the Mohs scale. It can be easily scratched by a copper coin. It has perfect rhombohedral cleavage, meaning it breaks into tilted blocks. One of its most famous properties is high birefringence (double refraction). If you place a clear calcite crystal (Iceland Spar) over text, the letters appear doubled.

Optical Use (Iceland Spar)

"Iceland Spar" is a chemically pure, transparent variety of calcite. It was used in World War II for gun sights and anti-aircraft weaponry because of its ability to polarize light. Today, synthetic calcite is still used in high-quality optical components and polarizers.

Colors & Varieties

Calcite occurs in every color. **Orange Calcite** is popular for carving. **Blue Calcite** is a soothing decorative stone. **Mangano Calcite** is pink due to manganese. **Dogtooth Spar** refers to sharp, pointy crystals. **Cobaltoan Calcite** is a vibrant, hot pink druzy variety found in Africa.

Acid Test

Calcite is the classic test subject for geology students. It reacts vigorously with dilute hydrochloric acid, fizzing and bubbling as it releases carbon dioxide gas. This simple test distinguishes it from quartz and other similar-looking white minerals.

Colors & Varieties

Colorless, white, yellow, red, orange, blue, green

Key Properties

  • Defining mineral for Hardness 3
  • Perfect rhombohedral cleavage
  • Strong double refraction
  • Effervesces (fizzes) in acid
  • Fluorescent (often red/pink)
  • Vitreous to pearly luster

Uses & Applications

  • Cement and concrete manufacturing
  • Soil conditioner (lime)
  • Optical polarizers
  • Ornamental stone
  • Pharmaceuticals (antacids)

Where to Find

  • USA (Missouri, Tennessee)
  • Mexico
  • Iceland (historic)
  • China
  • Brazil
  • Romania
  • Namibia

Frequently Asked Questions

Why does Calcite look doubled?

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This is due to birefringence, or double refraction. The crystal structure splits light into two separate rays traveling at different speeds. When these rays exit the crystal, they are in different positions, creating a double image of whatever is behind the stone.

Is Calcite a gemstone?

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Clear and colorful calcite is sometimes faceted for collectors, but it is generally too soft (hardness 3) and has too much cleavage for jewelry use. It would scratch and break almost immediately in a ring. Massive calcite (marble/onyx marble) is used for beads and carvings.

Does Calcite glow?

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Yes! Calcite is one of the most reliably fluorescent minerals. Many specimens from Franklin, New Jersey, glow a brilliant red under shortwave UV light. Other varieties can glow blue, yellow, or pink.

What is the difference between Calcite and Aragonite?

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They are polymorphs, meaning they have the same chemical formula (CaCO3) but different crystal structures. Calcite is trigonal and stable; Aragonite is orthorhombic and metastable (it will eventually turn into calcite over millions of years).

How do I clean Calcite?

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Use only water and a very mild soap. Never use vinegar or acidic cleaners, as they will literally dissolve the surface of the stone (releasing CO2 bubbles) and ruin the polish.

Related Minerals

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