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The kinetics of vaterite (CaCO 3 ) crystallization on calcite in the presence of aspartic acid was investigated by the constant composition technique. The presence of aspartic acid in the supersaturated solution stabilizes this calcium carbonated polymorph possibly through the development of active growth sites, which should show chemical and structural affinity to this mineral phase. The number of ions forming the critical nucleus was n* ) 4, and a surface energy of 31 mJ m -2 for the growing phase was estimated. The apparent order was found to be 1.75 ( 0.25, indicative for a surface diffusion controlled mechanism.

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The effect of sodium alginate on the crystal growth of hydroxyapatite (HAP) was investigated at sustained supersaturation by the constant composition technique. Sodium alginate was found to inhibit HAP crystal growth at low concentrations and reduced the crystal growth rates by 42-86% for inhibitor concentrations of 2.1x10(-7)-12.6x10(-7) mol/l. The inhibition effect on the crystal growth rate may be explained possibly through adsorption onto the active growth sites. A detailed kinetics analysis suggested a Langmuir-type adsorption of the alginate on HAP surface and a value of 1.63x10(7) l/mol was obtained for the affinity constant of sodium alginate for the surface of HAP. The apparent order for the crystallization reaction was determined to be approximately 2, thus suggesting a surface diffusion controlled spiral growth mechanism.