INVESTIGATING AN AMORPHOUS PRECURSOR PATHWAY TO CALCIFICATION: IMPLICATIONS FOR HIGH MAGNESIUM CARBONATES

With the realization that many calcified skeletons form by processes involving a precursor phase of amorphous calcium carbonate (ACC), a new paradigm for mineralization is emerging. There is evidence the Mg content in biogenic ACC is regulated by carboxylated (acidic) proteins and other macromolecules, but the physical basis for such a process is not well understood. In vitro experiments recently found that ACC compositions express a systematic relationship to the chemistry of carboxyl-rich biomolecules [1]. Molecules with a strong affinity for binding Ca compared to Mg promote the formation of Mg-enriched ACC products that are compositionally equivalent to high Mg-calcites and dolomite. Measurements show Mg/Ca ratios are controlled by a predictable dependence upon the binding properties of the organic molecules. The dependence suggests a physical basis for reports that specific sequences calcification promoting biomolecules are critical in modulating mineralization and controlling elemental signatures in carbonate skeletons. Insights from this study raise the long-standing question of why some geological carbonates often contain much higher Mg signatures than are possible by classical crystal growth processes in the laboratory. By analogy to calcification processes in organisms, we hypothesize that high Mg carbonates are formed by a non-classical process involving the transformation of these amorphous phases to the final crystalline product.

[1] Wang et al. (2009) PNAS 106. 21511-21516.