sedimentary

Chalk

Calcium Carbonate (CaCO3)

Hardness: 1-3 on Mohs scale; Color: white to light gray; Luster: dull/earthy; Crystal structure: Trigonal (microscopic); Cleavage: none; Specific gravity: 2.3-2.6

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

Identified March 20, 2026More sedimentary →

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Physical properties

Hardness: 1-3 on Mohs scale; Color: white to light gray; Luster: dull/earthy; Crystal structure: Trigonal (microscopic); Cleavage: none; Specific gravity: 2.3-2.6

Formation & geological history

Formed from the accumulation of microscopic skeletal remains from marine algae called coccolithophores. These sediments deposited in deep marine environments during the Cretaceous period (approx. 145 to 66 million years ago).

Uses & applications

Used in agriculture to raise soil pH, in the construction of blackboard chalk and lime, as a component in dental products, and occasionally as a fine abrasive.

Geological facts

The famous White Cliffs of Dover in England are made entirely of chalk. It is a highly porous rock and can actually act as a reservoir for natural gas and petroleum.

Field identification & locations

Identify in the field by its soft, powdery texture that easily rubs off on hands or other surfaces. It will vigorously effervesce (fizz) when exposed to a drop of dilute hydrochloric acid. Commonly found in coastal areas that were once prehistoric seabeds.