sedimentary

Chalk (Sedimentary Limestone)

Calcite (CaCO3)

AI-generated identification · may be incorrect

Hardness: 1-3 (Mohs scale), Color: White to greyish-white, Luster: Dull/Earthy, Crystal Structure: Hexagonal (microscopic), Cleavage: None visible in hand specimens, Specific Gravity: 2.2-2.3

Identified June 28, 2026More sedimentary →

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Physical properties

Hardness: 1-3 (Mohs scale), Color: White to greyish-white, Luster: Dull/Earthy, Crystal Structure: Hexagonal (microscopic), Cleavage: None visible in hand specimens, Specific Gravity: 2.2-2.3

Formation & geological history

Formed from the accumulation of microscopic skeletal remains from marine organisms (coccolithophores) that lived in the upper ocean. Most significant deposits date to the Cretaceous period.

Uses & applications

Used in the manufacturing of cement, lime, and paper. Also used in blackboard chalk (though modern chalk is often gypsum-based), toothpaste, and as an antacid in medicine.

Geological facts

The famous White Cliffs of Dover are composed almost entirely of chalk. The name 'Cretaceous' period comes from the Latin word 'creta,' meaning chalk.

Field identification & locations

Identified by its soft, powdery texture, white color, and vigorous reaction (effervescence) when it comes into contact with dilute hydrochloric acid. Commonly found in thick marine sequences.