Summary: The oriented transformation of aragonite to calcite has been studied by X-rays. The [100], [110], and [001] directions ofaragonite become [101¯0], [011¯0], and [0001] of calcite. The transformation was completed by a re-orientation of the CO₃ groups about the [001] direction of aragonite through an angle of 30°, different rows of CO₃ being turned in opposite directions, together with a displacement along the same direction. The layers of Ca ions also are shifted along the basal plane of calcite so that their arrangement perpendicular to the [0001] direction of calcite becomes ABCABCA… instead of ABABABA… as in aragonite.