a semihard, orthorhombic mineral resembling calcite, made up of calcium carbonate, CaCO: it is found in sedimentary and metamorphic rocks and in the skeletons of marine organisms
See aragonite in American Heritage Dictionary 4
(ə-răgˈə-nītˌ, ărˈə-gə-)
noun
An orthorhombic mineral form of crystalline calcium carbonate, dimorphous with calcite.