Aragonite.

Aragonite.

Aragonite, Calcium carbonate, CaCO3  has the same chemical composition as calcite, and vaterite. These three minerals are polymorphs of calcium carbonate, each with a different crystal lattice, so each one with a different crystal habit. Aragonite crystals are often pseudohexagonal and prismatic, as are the twinned crystals in this specimen from Morocco. Aragonite is not a stable polymorph, and will eventually  alter to calcite.