This paper reports a study of the spontaneous precipitation of calcium carbonate from aqueous solutions which are very supersaturated with respect to calcite, both in the absence of inhibitors, and in the presence of triphosphate as inhibitor. The sequence of events during precipitation is governed strongly by the initial supersaturation, t h e temperature, and t h e presence or absence of triphosphate.At high supersaturation, the first-formed phase is an amorphous calcium carbonate. It is observed only above a well defined ionic activity product, is homogeneously nucleated, and its formation is not inhibited by triphosphate. Over the temperature range 289-333 K, this amorphous phase has a solubility product, K , , defined by the equation : log K1 = (1247.0/T) -10.224At concentrations insufficient to produce the amorphous phase, t h e first-formed solid nucleates heterogeneously. In t h e absence of triphosphate it is calcite. In the presence of triphosphate, the first solid is calcium
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