Structural Chemistry & Crystallography Communication

Layered Structures in Biogenic Magnesium Calcite Single Crystals

Some calcitic biominerals have been shown to be formed through the crystallization of a Mg-rich amorphous calcium carbonate (Mg-ACC) precursor. Formation mechanisms of multiscale biogenic single crystal structures at ambient temperatures, in conditions of limited solid phase diffusion, remain largely incomprehensible. Based on experimental results, we develop a model describing the formation of the brittle star Mg-calcite nanostructure from an amorphous Mg-ACC precursor to a Mg-calcite nanostructure containing periodic layers with varying concentrations of coherent Mg-rich nano-inclusions. The formation route is rationalized in a two-step model: the first step involves spinodal decomposition of a liquid or gel-like Mg-ACC precursor into Mg-rich nanoparticles and a Mg-depleted amorphous matrix. The second step is the crystallization of the decomposed Mg-ACC precursor. The crystallization of Mg-depleted ACC matrix is accompanied by the exclusion of Mg ions into a diffusion zone adjacent to the crystallization front; after crystallization of a certain layer, the Mg concentration ahead of the layer exceeds the critical value above which the Mg-ACC matrix becomes unstable against spinodal decomposition. A secondary spinodal decomposition will then start and result in the formation of additional Mg-rich nano-domains. As a result, the density of Mg-rich nano-domains changes periodically and forms periodic layered structure inside magnesium calcite single crystals. The model was supported by our experimental results in synthetic Mg-calcite, which suggest a spinodal decomposition in the amorphous precursor.  These new insights have significant implications for fundamental understanding of the role of Mg-ACC material transformation during crystallization and its subsequent stability.

Author(s):

Alexander Katsman

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