sedimentary
Chalk
Carbonate rock (mainly CaCO3)
Hardness: 1-3 (Mohs scale); Color: White to light gray or buff; Luster: Dull/Earthy; Crystal Structure: Microcrystalline (Trigonal); Cleavage: None/Conchoidal; SG: 2.2-2.8
- Hardness
- 1-3 (Mohs scale)
- Color
- White to light gray or buff
- Luster
- Dull/Earthy
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Physical properties
Hardness: 1-3 (Mohs scale); Color: White to light gray or buff; Luster: Dull/Earthy; Crystal Structure: Microcrystalline (Trigonal); Cleavage: None/Conchoidal; SG: 2.2-2.8
Formation & geological history
Formed in deep marine environments from the accumulation of microscopic skeletons (coccoliths) of planktonic algae during the Cretaceous period.
Uses & applications
Used in agriculture to raise soil pH, in construction for cement manufacturing, as a writing material (blackboard chalk), and as a filler in paints and rubber.
Geological facts
The famous White Cliffs of Dover are the world's most iconic chalk formation. Most 'chalk' used in schools today is actually made from gypsum.
Field identification & locations
Identified by its soft, powdery texture and rapid effervescence (fizzing) when touched with dilute hydrochloric acid. It is very light and easily leaves a white streak.
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