Precipitation diagram of calcium carbonate polymorphs: its construction andsignificance

Abstract

In order to interpret the formation mechanism of calcium carbonate polymorphs, wepropose and construct a new ‘precipitation diagram’, which has two variables: the drivingforce for nucleation and temperature. The precipitation experiments were carriedout by mixing calcium chloride and sodium carbonate aqueous solutions. As aresult, a calcite–vaterite co-precipitation zone, a vaterite precipitation zone, avaterite–aragonite co-precipitation zone and an aragonite precipitation zone canbe defined. Theoretical considerations suggest that the steady state nucleationtheory can explain well the appearance of these four zones, and the first-orderimportance of the temperature dependency of surface free energy in the nucleationof aragonite. Furthermore, the addition of an impurity will likely result in thechange of these energies, and this precipitation diagram gives a new basis forinterpreting the nature of the polymorphs precipitated in both inorganic and biologicalenvironments.