sedimentary
Chalk (Sedimentary Limestone)
Calcium Carbonate (CaCO3) - Bioclastic Limestone
Hardness: 1-3 (Mohs scale), Color: White to light gray or yellowish buff, Luster: Dull/Earthy, Crystal Structure: Trigonal (microscopic calcite), Cleavage: None (conchoidal to irregular fracture), Specific Gravity: 2.2-2.8
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Physical properties
Hardness: 1-3 (Mohs scale), Color: White to light gray or yellowish buff, Luster: Dull/Earthy, Crystal Structure: Trigonal (microscopic calcite), Cleavage: None (conchoidal to irregular fracture), Specific Gravity: 2.2-2.8
Formation & geological history
Formed from the accumulation of microscopic marine organisms like coccolithophores over millions of years. Most common deposits date to the Cretaceous period (roughly 66-145 million years ago) in deep marine environments.
Uses & applications
Used in the manufacturing of portland cement, as a soil conditioner to raise pH, in blackboard chalk (though modern chalk is often gypsum), and as a filler in paper and paints.
Geological facts
Chalk form massive cliffs, the most famous being the White Cliffs of Dover in England. It is a porous rock that can hold significant amounts of groundwater, serving as an important aquifer.
Field identification & locations
Identify in the field by its softness (it will leave a white streak on harder surfaces), light weight, and strong reaction (fizzing) when exposed to a drop of dilute hydrochloric acid. Commonly found in coastal areas or prehistoric sea beds.
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