sedimentary

Chalk

Calcium Carbonate (CaCO3)

Hardness: 1-3 (Mohs scale); Color: White to light grey or buff; Luster: Dull/Earthy; Crystal Structure: Microcrystalline (trigonal); Cleavage: None; Specific Gravity: 2.2-2.3

Hardness: 1-3 (Mohs scale)
Color: White to light grey or buff
Luster: Dull/Earthy

Identified March 22, 2026More sedimentary →

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Physical properties

Hardness: 1-3 (Mohs scale); Color: White to light grey or buff; Luster: Dull/Earthy; Crystal Structure: Microcrystalline (trigonal); Cleavage: None; Specific Gravity: 2.2-2.3

Formation & geological history

Formed in deep marine environments through the accumulation of microscopic skeletal remains of planktonic organisms (coccoliths). Primarily Cretaceous in age (late Mesozoic).

Uses & applications

Used in the production of quicklime, cement, fertilizer, and blackboard chalk. Also used in pharmaceuticals (antacids) and as a white pigment in paper and rubber.

Geological facts

The famous White Cliffs of Dover in England are composed almost entirely of chalk. It is a highly porous form of limestone that can hold significant amounts of water or oil in its pore spaces.

Field identification & locations

Identify by its soft, powdery texture and reactiveness to dilute hydrochloric acid (effervescence). Found worldwide in former marine basins like Western Europe and the Great Plains of the US.