How to identify Calcite

To correctly identify Calcite (Calcite (Calcium Carbonate - CaCO3)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to sub-vitreous, sometimes pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, fibrous, stalactitic, botryoidal, or crystalline. Crystals can be blocky, tabular, prismatic, scalenohedral, or rhombohedral.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect rhombohedral cleavage in three directions, producing characteristic rhomb-shaped fragments. The cleavage angles are approximately 74° and 106°.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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

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