THE TRANSFORMATION OF AMORPHOUS CALCIUM CARBONATE TO CALCITE IN THE PRESENCE OF STRONTIUM

The transformation pathway and crystallization rates of amorphous calcium carbonate (ACC) to crystalline calcium carbonate as part of biomineralization processes are controlled by numerous factors, including temperature, pH, and foreign ions (e.g., Sr²⁺). The presence of Sr is known to inhibit single-crystal calcite growth. However, it is not clear how Sr²⁺ affect ACC precursor to crystalline CaCO₃ formation. In this study, we investigated the transformation of ACC to calcite (CaCO₃) with the presence of Sr²⁺ in aqueous conditions to understand the transformation pathway and mechanisms.

Sr²⁺-free ACC materials with stoichiometry of Ca_1.000(OH)_0.098(CO₃)_0.951·1.246H₂O were synthesized and confirmed by thermogravimetric analysis and X-ray diffraction (XRD). To study the Sr²⁺ incorporation effects during ACC to calcite transformation, 200 mg of Sr-free ACC powers were mixed in 20 mL SrCl₂ solution with Sr²⁺ concentration varied from 0-0.1M at 25^oC in batch reactors. The solid end products were analyzed ex-situ with SEM-EDS, XRD, and Raman spectroscopy for every 5, 10, 30, 180, and 1440 minutes. When the initial Sr²⁺ concentration is higher than 0.8 mM (i.e., Sr/Ca > 0.01), a mixture of calcite and strontianite (SrCO₃) is formed. When Sr²⁺ concentration is less than 0.8 mM (i.e., Sr/Ca < 0.01), the final product is found to be a single phase of calcite, but with up to 10 wt.% Sr in replacement of Ca in the calcite structure. This latter observation can be explained by dissolution and reprecipitation reactions in which ACC dissolves first and reprecipitates to form Sr-rich calcite. In addition, the crystallinity and crystal morphology of calcite are affected by the enrichment of Sr²⁺ in the initial solution. For instance, with higher Sr²⁺ concentration, calcite developed more multi-twinned crystals and reduced crystallinity visible in peak broadening of Raman spectra. Significant Raman peak shift of the C-O in-plane bending mode around 712 cm^-1 and rotational mode around 280 cm^-1 to the lower frequencies is observed, which suggests positional disorder of carbonate ions and induced-lattice strains due to the Sr²⁺ to Ca²⁺ substitution. Details about kinetics and crystallographic changes during ACC to calcite transformations will be studied in the near future by in-situ time-resolved synchrotron XRD methods.