sedimentary

Chalk

Calcium Carbonate (CaCO3)

Hardness: 1-2.5 on Mohs scale; Color: typically white to light grey; Luster: dull/earthy; Crystal structure: trigonal (microscopic); Cleavage: none; Specific gravity: 2.2-2.3.

Hardness: 1-2
Color: typically white to light grey
Luster: dull/earthy

Identified June 1, 2026More sedimentary →

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Physical properties

Hardness: 1-2.5 on Mohs scale; Color: typically white to light grey; Luster: dull/earthy; Crystal structure: trigonal (microscopic); Cleavage: none; Specific gravity: 2.2-2.3.

Formation & geological history

Formed from the gradual accumulation and compression of the skeletal remains (coccoliths) of microscopic marine algae over millions of years in deep marine environments, primarily during the Cretaceous period.

Uses & applications

Used in the manufacture of lime, cement, and blackboard chalk; as a filler in paper, plastics, and paint; in agriculture to treat acidic soils; and in sports for grip.

Geological facts

The famous White Cliffs of Dover are made entirely of chalk. It is a soft, porous form of limestone. Despite its name, modern blackboard chalk is often made from mineral gypsum (calcium sulfate).

Field identification & locations

Identify in the field by its white color, soft/powdery texture, and energetic effervescence when exposed to a drop of hydrochloric acid. Commonly found in regions with prehistoric seafloors like the UK, France, and parts of the US.