Following Shiraki and Brantley (1995), who found different growth mechanisms for calcite crystal growth at 100°C and 100 bars total pressure, we investigated the inhibition of calcite crystal growth by Mg 2+ under the same conditions of temperature, pressure, and composition. The results showed that the Mg 2+ inhibition depends on the growth regime. For growth in the exponential rate regime, the growth of calcite is almost totally inhibited, and aragonite grows instead. For growth in the linear rate regime, both aragonite and calcite precipitate. On the other hand, for experiments best described by a parabolic growth model, Mg inhibits the growth rate of calcite, but no aragonite precipitates. The study also showed that Mg inserts into the calcite structure but not into the aragonite structure to any significant extent during growth. The aragonite growth, which is controlled by primary nucleation, suggests that neither nucleation nor growth are inhibited by Mg. Discrepant observations concerning Mg inhibition documented in the literature may be explained by such differences in the growth mechanism.