sedimentary

Chalk (Limestone)

Calcium Carbonate (CaCO3)

Hardness: 1-3 on Mohs scale; Color: white to light grey; Luster: dull/earthy; Texture: fine-grained, soft, and porous; Fracture: conchoidal or uneven.

Hardness: 1-3 on Mohs scale
Color: white to light grey
Luster: dull/earthy

Identified July 4, 2026More sedimentary →

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Physical properties

Hardness: 1-3 on Mohs scale; Color: white to light grey; Luster: dull/earthy; Texture: fine-grained, soft, and porous; Fracture: conchoidal or uneven.

Formation & geological history

Formed from the accumulation of microscopic marine organisms like coccolithophores. It typically dates to the Cretaceous period, formed in deep marine environments.

Uses & applications

Used in the production of lime, cement, and blackboard chalk. It is also used as a filler in paper, plastics, and toothpaste, and as a soil conditioner in agriculture.

Geological facts

The famous White Cliffs of Dover in England are composed entirely of chalk. The name 'Cretaceous' period actually comes from 'creta', the Latin word for chalk.

Field identification & locations

Identified by its softness (it leaves a white streak on surfaces) and its reaction to acid (it fizzes vigorously when in contact with diluted HCl). Often found in coastal regions or areas that were ancient seabeds.