limestone - teaching hand specimen of a tan silty limestone
This tan silty limestone is interbedded with shales in the Middle Jurassic Carmel Formation in southwestern Utah. Limestones are primarily biogenic in origin, being composed of the protective shells of small organisms that extract calcium carbonate from seawater to build their shells. There is a constant rain of microscopic shells and other calcareous parts onto the sea floor, which when accumulated and consolidated becomes limestone.
Chalk is a porous form of limestone composed entirely of minute calcium carbonate shells of coccolithophores, a unicellular phytoplankton. Coquina is a form composed of shells and shell fragments, only partly consolidated. When a limestone contains clay, it grades towards shale and is called an argillaceous limestone.
Because the composition of limestone is calcium carbonate, a limestone will strongly effervesce with dilute hydrochloric acid. Dolomite, which can have the same appearance as a limestone, is calcium-magnesium-carbonate, and effervesces only reluctantly. Often a dolomite will effervesce with acid only where powdered by a few taps of a geologist's hammer, a useful field test. This limestone is a good contrast with the more typical gray limestones. Students should understand that limestone is not always gray, and that it can vary in color.
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