sedimentary

Chalk

Calcium Carbonate (CaCO3)

Hardness: 1-3 Mohs; Color: White to light gray; Luster: Dull, earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.3-2.6

Hardness: 1-3 Mohs
Color: White to light gray
Luster: Dull, earthy

Identified July 9, 2026More sedimentary →

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Physical properties

Hardness: 1-3 Mohs; Color: White to light gray; Luster: Dull, earthy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.3-2.6

Formation & geological history

Formed from the accumulation of microscopic marine organisms known as coccolithophores. These calcareous remains settle on the ocean floor and are lithified over millions of years, primarily during the Cretaceous period.

Uses & applications

Used in the manufacturing of lime, cement, and concrete; as a filler in paper, paints, and plastics; in agriculture to treat acidic soils; and traditionally for writing tools.

Geological facts

The famous White Cliffs of Dover in England are composed almost entirely of chalk. It is a soft, porous form of limestone that is highly reactive to acids.

Field identification & locations

Identify by its soft, powdery texture and its ability to easily leave a white mark on dark surfaces. It will vigorously effervesce (fizz) when in contact with dilute hydrochloric acid.