Senior Mineralogist, Geological Survey of India,

Summary. The oriented transformation f aragonite to calcite has been studied by X-rays. The [100], [ll0], and [001] directions ofaragonite become [1010], [0110], and [0001] of calcite. The transformation was completed by a re-orientation f the CO a groups about he [00l] direction of aragonite through an angle of 30 ~ different rows of CO s 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. T HE anhydrous carbonates of metals may be divided into two groups: CaCO 8 exists in both groups as calcite and aragonite. Divalent metals having smaller ionic radii than that of calcium belong to the calcite series. The others, with greater ionic radii, are members of the aragonite series. The structures of calcite and aragonite were analysed by Bragg (1914, 1924). In calcite, the Ca ions are arranged in a cubic close-packed (ABCABCA...) way in layers perpendicular to the threefold axis of the rhombohedral cell. Aragonite, on the other hand