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A usually white, yellowish, or pink orthorhombic mineral that can occur in many different colors. Aragonite occurs as acicular (needlelike) or tabular crystals, or as fibrous aggregates. It is found in gypsum deposits, at the tips of calcite crystals, in mollusk shells and pearls, and in living reef structures. It is a polymorph of calcite. Chemical formula: CaCO3.