sedimentary
Chalk (Calcitic)
Calcium Carbonate (CaCO3) - Biogenic Sedimentary Rock
Hardness: 1-3 (Mohs scale); Color: White to light grey; Luster: Dull or earthy; Crystal structure: Microcrystalline (Trigonal/Hexagonal); Cleavage: None/Earthy; Specific gravity: 2.3-2.6
- Hardness
- 1-3 (Mohs scale)
- Color
- White to light grey
- Luster
- Dull or earthy
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Physical properties
Hardness: 1-3 (Mohs scale); Color: White to light grey; Luster: Dull or earthy; Crystal structure: Microcrystalline (Trigonal/Hexagonal); Cleavage: None/Earthy; Specific gravity: 2.3-2.6
Formation & geological history
Formed in deep marine environments from the accumulation of microscopic skeletal remains (coccoliths) of planktonic algae. Most prominent deposits date to the Cretaceous Period (145-66 million years ago).
Uses & applications
Used in the manufacturing of lime, cement, and concrete. Formerly used extensively for school blackboards (now mostly replaced by gypsum) and as a pigment or filler in paints and plastics.
Geological facts
The 'White Cliffs of Dover' in England are the most famous chalk formation in the world. It is a highly porous rock that can hold significant amounts of water or oil in underground reservoirs.
Field identification & locations
Identify by its soft texture (can often be scratched with a fingernail), white rub-off on hands, and vigorous reaction (fizzing) when exposed to a drop of weak acid. Commonly found in coastal areas or formerly submerged basins.
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