The mechanics of ion incorporation into a crystal are examined in order to evaluate mechanisms of calcite growth inhibition and morphology control. Crystallographic considerations indicate that while the Folk (1974) mechanisms of Mg (super +2) interaction with calcite may be responsible for inhibiting crystal growth, it is inadequate to explain morphologic variation. Experimental evidence on the kinetics of calcite crystal growth indicate that dewatering of surface adsorbed Ca (super +2) ions may be the rate-determining step in calcite growth, thus supporting, by analogy, the heat of hydration mechanism for Mg (super +2) poisoning of calcite growth. Surface charge and colloid chemical considerations indicate that differential charge development on crystal faces is responsible for morphologic variation in calcite. The surface charge model for morphology control successfully predicts the approximate Ca (super +2) /CO 3 (super -2) ratio of water yielding equant calcite crystals and the morphology of aragonite crystals precipitated from sea water.