sedimentary

Chalk

Calcium Carbonate (CaCO3)

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

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

Identified March 23, 2026More sedimentary →

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Physical properties

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

Formation & geological history

Formed in deep marine environments from the accumulation of microscopic skeletal remains (coccoliths) and shells of marine organisms like foraminifera. Most notable deposits date back to the Cretaceous period (approx. 100 to 65 million years ago).

Uses & applications

Historically used for writing tools. Industrially used in the production of lime, cement, blackboard chalk (organic), and as a soil conditioner in agriculture to raise pH levels.

Geological facts

The White Cliffs of Dover in England are the world's most famous chalk formation. While modern blackboard 'chalk' is often made from powdered gypsum, true geological chalk is a specific form of limestone.

Field identification & locations

Identify in the field by its soft, powdery texture that leaves a white residue on fingers and reacts vigorously (fizzes) with cold diluted hydrochloric acid. Commonly found in coastal cliff regions and inland sedimentary basins.