sedimentary
Chalk / Sedimentary Limestone
Calcium Carbonate (CaCO3) - Biogenic Sedimentary Rock
Hardness: 1-3 on Mohs scale (very soft, can be scratched with a fingernail); Color: white to off-white/cream; Luster: dull, earthy; Crystal structure: microcrystalline (fine-grained); Cleavage: none; Specific gravity: 1.9 - 2.8; Textural note: highly porous and friable.
- Color
- white to off-white/cream
- Luster
- dull, earthy
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Physical properties
Hardness: 1-3 on Mohs scale (very soft, can be scratched with a fingernail); Color: white to off-white/cream; Luster: dull, earthy; Crystal structure: microcrystalline (fine-grained); Cleavage: none; Specific gravity: 1.9 - 2.8; Textural note: highly porous and friable.
Formation & geological history
Formed from the gradual accumulation of calcitic microscopic remains of marine organisms, such as coccolithophores, on the seafloor over millions of years (typically Cretaceous period). These layers are eventually compacted into soft, porous rock.
Uses & applications
Used in the manufacturing of portland cement, lime, and quicklime. It is historically significant for writing/drawing, and is used as a pH neutralizer in agriculture, an abrasive in toothpaste, and a filler in paper and plastics.
Geological facts
The famous 'White Cliffs of Dover' in England are composed entirely of this material. Despite its soft appearance, chalk is remarkably resistant to erosion compared to some clays, allowing it to form towering coastal cliffs.
Field identification & locations
Identify by its ability to leave a white streak on dark surfaces, its softness (it will crumble if hit), and its immediate effervescence (bubbling) when a drop of weak hydrochloric acid or vinegar is applied. Found globally in areas that were once shallow seas.
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