sedimentary
Chalk
Calcium Carbonate (CaCO3)
Hardness: 1 on Mohs scale; Color: White to off-white/light grey; Luster: Dull or earthy; Structure: Microcrystalline/non-crystalline appearance; Cleavage: None (conchoidal fracture); Specific Gravity: 2.3-2.6
- Hardness
- 1 on Mohs scale
- Color
- White to off-white/light grey
- Luster
- Dull or earthy
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Physical properties
Hardness: 1 on Mohs scale; Color: White to off-white/light grey; Luster: Dull or earthy; Structure: Microcrystalline/non-crystalline appearance; 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 (coccoliths) of plankton. Most deposits date to the Cretaceous period.
Uses & applications
Used in agriculture to raise soil pH, in the manufacture of cement, as a filler in paper and plastics, and historically for writing or drawing.
Geological facts
The famous White Cliffs of Dover are composed almost entirely of chalk. It is a highly porous form of limestone that can hold significant amounts of water or oil in underground reservoirs.
Field identification & locations
Identify by its extreme softness (can be scratched by a fingernail), white rub-off on hands, and violent effervescence when exposed to dilute hydrochloric acid. Commonly found in coastal cliff areas or inland quarries.
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