Vaterite-calcite transition

An example of an intensively studied set of polymorphs whose decompositions are of great theoretical and practical importance (see Chapter 12) is CaCOj which may exist (in order of decreasing thermodynamic stability) as calcite, aragonite or vaterite [18]. Vaterite can be prepared by precipitation from aqueous solutions under carefully controlled conditions. A DTA curve for the vaterite calcite transition is shown in Figure 2.3. The transition is exothermic AH = -34.3 J g ) with onset at 704 K. Isothermal extent of conversion against time curves were described [18] by the Johnson, Mehl, Avrami, Erofeev model (see Chapter 3) with n = 2. The measured Arrhenius parameters were F, = 210 kJ mol and A = 1.15x10 min. The decomposition of vaterite and its concurrent transformation to calcite under various conditions were compared [18] with the decomposition of calcite xmder the same conditions (see Chapter 12). 

DTA curve for the vaterite - calcite transition at a heating rate of 10 K min in flowing argon (reproduced from [18] with permission). 

An additional possible reason for differences in behaviour is that CaCOj exists in three crystalline forms. Calcite is the most stable and aragonite and vaterite undergo transitions to calcite at 728 K and between 623 and 673 K, respectively. Consequently, at decomposition temperatures (above 900 K) the reactant CaCOj would normally be in the calcite structure [2]. Salvador et al. [3] concluded that the... 

From the more recent reports cited below, further references to the extensive literature concerned with calcite decomposition may be traced. Other modifications of CaC03 (aragonite and vaterite) undergo solid phase transitions to calcite at temperatures of 728 K and 623—673 K respectively [733], below those of onset of decomposition (>900 K). There is strong evidence [742] that the reaction... 

A third polymorph of CaCOs, vaterite, may form as an inorganic precipitate, but is most likely to be encountered as the mineral forming in the nacre of mollusks (Section 8.04.3.4.6). The mineral crystallizes in the hexagonal/rhombohedral space group P63/mmc, and the calcium is in sixfold coordination (Gaines et al., 1997, p. 440). It is unstable and often reverts to calcite over time. Elevated temperatures or water high in NaCl will accelerate the transition in dry environments, vaterite will slowly revert to calcite. 

Davies et al. (174) found that the polymorph transition kinetics of the changes of vaterite to calcite and aragonite to calcite could be followed using DTA and the equation... 

The polymorphism of calcium carbonate is still not completely understood despite its very long history and the appearance of many published studies. In addition to calcite, aragonite and vaterite, non-crystalline forms of CaCOs of biolo cal origin exist. An account of the relationships between these solid phases has appeared recently [39] together with a summary of thermodynamic and kinetic data for the transformations of the metastable polymorphs aragonite and vaterite to the stable calcite. These authors describe the preparation of non-crystalline calcium carbonate and report preliminary values of the transition temperature and enthalpy change for its crystallization to calcite. The DSC method, siq)ported by TG and PXRD, was used in this study. 

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