sedimentary
Chalk
Micrite (primarily Calcium Carbonate, CaCO3)
Hardness: 1-3 (Mohs scale); Color: White to light grey; Luster: Dull/Earthy; Structure: Fine-grained, porous; Cleavage: None; Specific Gravity: 2.4-2.6
- Hardness
- 1-3 (Mohs scale)
- Color
- White to light grey
- Luster
- Dull/Earthy
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Physical properties
Hardness: 1-3 (Mohs scale); Color: White to light grey; Luster: Dull/Earthy; Structure: Fine-grained, porous; Cleavage: None; Specific Gravity: 2.4-2.6
Formation & geological history
Formed in deep marine conditions from the accumulation of microscopic skeletal remains of calcareous algae (coccoliths) and plankton. Primarily associated with the Cretaceous period.
Uses & applications
Used in the manufacturing of portland cement, lime, and bricks. Formerly used extensively as a writing medium on blackboards. Used in agriculture to treat acidic soils and in sports for better grip.
Geological facts
The famous White Cliffs of Dover in England are composed primarily of chalk. Most modern 'blackboard chalk' is actually made of gypsum, though historically it was made from natural chalk.
Field identification & locations
Identify in the field by its soft, powdery texture that easily rubs off on fingers and its vigorous reaction (effervescence) when touched with dilute hydrochloric acid. Commonly found in coastal areas or former seabed deposits.
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