How to identify Calcite

To correctly identify Calcite (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, yellow, green, blue, pink, brown, or black due to impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can range from fine-grained to coarse-grained, massive, granular, fibrous, or stalactitic. Individual crystals are often smooth and well-formed.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Highly variable, including rhombohedral, scalenohedral, prismatic, tabular, and acicular forms. Twinning is common.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect rhombohedral cleavage in three directions, producing characteristic rhomb-shaped fragments.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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

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