FORMATION OF HIGH MAGNESIUM CALCITE FROM AMORPHOUS CALCIUM CARBONATE IN INORGANIC AND CARBOXYL-RICH ENVIRONMENTS

Overwhelming evidence from structural biology shows the skeletons of many calcifying organisms begin with the accumulation of an amorphous calcium carbonate precursor (ACC) that subsequently transforms to calcite. This type of non classical mineralization process would explain reports that biogenic calcite can contain up to 30 mole % MgCO₃, in addition to the occurrence of some high-Mg carbonates that are found in modern sedimentary environments. These anomalously high amounts of Mg cannot be achieved through classical step growth.

This experimental study determined the Mg content of calcites that grew from an ACC precursor in the absence and presence of carboxylated biomolecules (aspartate, citrate, tartarate, and oxydiacetate). ACC was synthesized from solutions with initial Mg/Ca ratios that were varied from 0–15, a fixed initial total carbonate concentration (0.10 M NaHCO₃), and fixed initial pH (9.5) using a batch reactor method. The ACC was then allowed to transform within the mother solution. Results show the Mg contents in the crystal phases are positively correlated to Mg levels in the ACC precursors. For all solution compositions, ACC transforms into spherulitic calcite crystallites that contain 10–38 mole % MgCO₃, without evidence of secondary polymorphs. At low initial solution Mg/Ca ratios, more Mg is incorporated into the calcite crystal than in the ACC precursor, but the Mg content of ACC and the resulting calcite is similar in solutions with initial Mg/Ca ratios up to 5. Above a Mg/Ca ratio of approximately 8, however, the ACC transforms to calcite that excludes some of the Mg to produce (for these conditions) a maximum of approximately 30 mole % Mg (in inorganic solutions). The presence of citrate, tartarate or oxydiacetate significantly slows the transformation process by stabilizing ACC nanoparticles, presumably through their negative electrostatic field. In addition, the Mg contents in ACC and calcite crystals are organic-specific and correlate with the selectivity of carboxylates for Ca²⁺ over Mg²⁺ in aqueous solutions.