sedimentary
Chalk
Calcium Carbonate (CaCO3)
Hardness: 1-2.5 (Mohs), Color: White to off-white/gray, Luster: Dull/Earthy, Crystal Structure: Trigonal (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.3-2.6
- Hardness
- 1-2
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Physical properties
Hardness: 1-2.5 (Mohs), Color: White to off-white/gray, Luster: Dull/Earthy, Crystal Structure: Trigonal (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.3-2.6
Formation & geological history
Formed in deep marine environments from the accumulation of microscopic skeletal remains of calcareous algae (coccolithophores) during the Cretaceous period.
Uses & applications
Used in agriculture to raise soil pH, in the manufacturing of portland cement, as a whitening agent in toothpaste, and historically as a writing tool.
Geological facts
The White Cliffs of Dover in England are the most famous natural chalk formations. Chalk is a specific variety of limestone.
Field identification & locations
Identified by its soft, powdery texture that easily rubs off on fingers and its high reactivity to cold dilute hydrochloric acid (vigorous fizzing).
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