sedimentary
Chalk (Limestone variant)
Calcium Carbonate (CaCO3)
Hardness: 1-3 (Mohs scale); Color: White to light gray; Luster: Dull, earthy; Crystal structure: Trigonal (microscopic); Cleavage: None; Specific gravity: 2.4-2.6. Very porous and soft.
- Hardness
- 1-3 (Mohs scale)
- Color
- White to light gray
- Luster
- Dull, earthy
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Physical properties
Hardness: 1-3 (Mohs scale); Color: White to light gray; Luster: Dull, earthy; Crystal structure: Trigonal (microscopic); Cleavage: None; Specific gravity: 2.4-2.6. Very porous and soft.
Formation & geological history
Formed in deep marine environments from the accumulation of microscopic skeletal remains from coccolithophores (plankton). Most famous deposits date to the Cretaceous period (from 'creta', Latin for chalk).
Uses & applications
Used in the manufacture of lime, cement, and concrete; as a filler in paper and plastics; as a soil conditioner in agriculture; and historically for drawing/writing.
Geological facts
The White Cliffs of Dover are the most famous chalk formation in the world. Chalk is actually a type of limestone that is mostly composed of the mineral calcite.
Field identification & locations
Identified in the field by its soft, powdery texture and its vigorous reaction (effervescence) when a small amount of dilute hydrochloric acid is applied. It can easily leave a white streak on harder surfaces.
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