Calcium is a very abundant element, which in nature above all occurs as calcium carbonate (CaCO3, lime) and calcium sulfate (CaSO4, gypsum). Elemental calcium is a grey metal, that slowly reacts with air and fiercely reacts with water. For humans and animals it is first of all important, because bones, teeth and exoskeletons to a large part consist of calcium compounds like tricalcium phosphate and calcium carbonate.
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