sedimentary
Chalk (Sedimentary Limestone)
Calcite (CaCO3)
Hardness: 1-3 (Mohs scale), Color: White to greyish-white, Luster: Dull/Earthy, Crystal Structure: Hexagonal (microscopic), Cleavage: None visible in hand specimens, Specific Gravity: 2.2-2.3
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Physical properties
Hardness: 1-3 (Mohs scale), Color: White to greyish-white, Luster: Dull/Earthy, Crystal Structure: Hexagonal (microscopic), Cleavage: None visible in hand specimens, Specific Gravity: 2.2-2.3
Formation & geological history
Formed from the accumulation of microscopic skeletal remains from marine organisms (coccolithophores) that lived in the upper ocean. Most significant deposits date to the Cretaceous period.
Uses & applications
Used in the manufacturing of cement, lime, and paper. Also used in blackboard chalk (though modern chalk is often gypsum-based), toothpaste, and as an antacid in medicine.
Geological facts
The famous White Cliffs of Dover are composed almost entirely of chalk. The name 'Cretaceous' period comes from the Latin word 'creta,' meaning chalk.
Field identification & locations
Identified by its soft, powdery texture, white color, and vigorous reaction (effervescence) when it comes into contact with dilute hydrochloric acid. Commonly found in thick marine sequences.
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