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Limestone. Credit: Mineral Information Institute
A sedimentary rock composed mainly of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). The chief minerals found in limestone are calcite and aragonite; dolomite is also be present in the dolomitic limestones.

Of the many varieties of limestone most have been deposited in shallow water. Organic limestones (e.g., chalk) are formed from the calcareous skeletons of marine organisms. Precipitated limestones include oolite, which is composed of ooliths – spherical bodies formed by the precipitation of carbonate around a nucleus. Clastic limestones are derived from fragments of pre-existing calcareous rocks. Limestone is used as building stone, and in the manufacture of lime, carbon dioxide, and cement.

Quicklime (calcium oxide) is obtained from limestone in a lime kiln.

Many millions of tons of limestone are used every year as a 'flux' mixed with iron ore in blast furnaces. The limestone combines with impurities in the ore to form a slag.

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