sedimentary

Chalk

Calcium Carbonate (CaCO3)

Hardness: 1-2.5 on Mohs scale; Color: typically white to light gray; Luster: dull, earthy; Crystal structure: Trigonal; Cleavage: none; Specific gravity: 2.3-2.6

Hardness: 1-2
Color: typically white to light gray
Luster: dull, earthy

Identified June 9, 2026More sedimentary →

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Physical properties

Hardness: 1-2.5 on Mohs scale; Color: typically white to light gray; Luster: dull, earthy; Crystal structure: Trigonal; Cleavage: none; Specific gravity: 2.3-2.6

Formation & geological history

Formed in deep marine conditions from the accumulation of microscopic skeletal debris (coccoliths) of planktonic algae over millions of years, primarily during the Cretaceous period.

Uses & applications

Historically used for writing on blackboards; currently used in agriculture to raise soil pH, in construction for cement, and in medicines as an antacid.

Geological facts

The famous White Cliffs of Dover in England are composed entirely of chalk, formed by the remains of billions of microscopic marine organisms.

Field identification & locations

Identify by its soft, powdery texture that easily leaves a white mark on surfaces and its vigorous effervescence when exposed to cold dilute hydrochloric acid.