sedimentary

Chalk (Limestone variant)

Calcium Carbonate (CaCO3)

AI-generated identification · may be incorrect

Hardness: 2.5-3 (Mohs scale), Color: White to light grey or tan, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.3-2.6

Hardness: 2

Identified March 30, 2026More sedimentary →

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Physical properties

Hardness: 2.5-3 (Mohs scale), Color: White to light grey or tan, Luster: Dull/Earthy, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal or irregular fracture), Specific Gravity: 2.3-2.6

Formation & geological history

Formed from the accumulation of microscopic marine organisms like coccolithophores over millions of years. Most large deposits date back to the Cretaceous period (approx. 145 to 66 million years ago).

Uses & applications

Used in agriculture to raise soil pH, in the construction of cement and mortar, and historically as a writing medium and blackboard chalk.

Geological facts

The 'White Cliffs of Dover' in England are the most famous geological example of chalk. It is highly porous and can act as a massive natural carbon sink.

Field identification & locations

Identify in the field by its soft, powdery texture that easily rubs off on fingers and it will vigorously effervesce (bubble) when a drop of diluted hydrochloric acid is applied. Commonly found in coastal bluffs and quarries.