An assessment of calcite crystal growth mechanisms based on crystal size distributions

Calcite crystal growth experiments were undertaken to test a recently propo sed model that relates crystal growth mechanisms to the shapes of crystal s ize distributions (CSDs). According to this approach, CSDs for minerals hav e three basic shapes: (1) asymptotic, which is related to a crystal growth mechanism having constant-rate nucleation accompanied by surface-controlled growth; (2) lognormal, which results from decaying-rate nucleation accompa nied by surface-controlled growth; and (3) a theoretical, universal, steady -state curve attributed to Ostwald ripening. In addition, there is a fourth crystal growth mechanism that does not have a specific CSD shape, but whic h preserves the relative shapes of previously formed CSDs. This mechanism i s attributed to supply-controlled growth. All three shapes were produced experimentally in the calcite growth experim ents by modifying nucleation conditions and solution concentrations. The as ymptotic CSD formed when additional reactants were added stepwise to the su rface of solutions that were supersaturated with respect to calcite (initia l Omega = 20, where Omega = 1 represents saturation), thereby leading to th e continuous nucleation and growth of calcite crystals. Lognormal CSDs resu lted when reactants were added continuously below the solution surface, via a submerged tube, to similarly supersaturated solutions (initial Omega = 2 2 to 41), thereby leading to a single nucleation event followed by surface- controlled growth. The Ostwald CSD resulted when concentrated reactants wer e rapidly mixed, leading initially to high levels of supersaturation (Omega >100), and to the formation and subsequent dissolution of very small nucle i, thereby yielding CSDs having small crystal size variances. The three CSD shapes likely were produced early in the crystallization proc ess, in the nanometer crystal size range, and preserved during subsequent g rowth. Preservation of the relative shapes of the CSDs indicates that a sup ply-controlled growth mechanism was established and maintained during the c onstant-composition experiments. CSDs having shapes intermediate between lo gnormal and Ostwald also were generated by varying the initial levels of su persaturation (initial Omega = 28.2 to 69.2) in rapidly mixed solutions. Lognormal CSDs were observed for natural calcite crystals that are found in septarian concretions occurring in southeastern Colorado. Based on the mod el described above, these CSDs indicate initial growth by surface control, followed by supply-controlled growth. Thus, CSDs may be used to deduce crys tal growth mechanisms from which geologic conditions early in the growth hi story of a mineral can be inferred. Conversely, CSD shape can be predicted during industrial crystallization by applying the appropriate conditions fo r a particular growth mechanism. Copyright (C) 2000 Elsevier Science Ltd.