Pressure-Induced Polyamorphism and Formation of 'Aragonitic' Amorphous Calcium Carbonate

摘要

Amorphous calcium carbonate (ACC) is a precursor to the crystalline phases of CaCO3, commonly found in the earliest stages of biomineral development and as one of the metastable states formed during the inorganic precipitation of calcium carbonate crystalline polymorphs. Its isotropic and hydrous moldable character allows many organisms to form very complex conformations of their shells or skeletons by taking advantage of these unique properties. At least two different phases of biogenic ACC have been described to date: a highly hydrated phase with one water molecule per CaCO3 unit, and an anhydrous phase that forms as a transient phase prior to crystallization to vaterite or calcite. Recently, the existence of polyamorphism (the existence of a substance in different amorphous modifications, akin to polymorphism in crystalline materials) in synthetic hydrated ACC has been suggested based mainly on X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR) data that show different local structures of ACC precipitated from solutions at different pH values: calcite-like ACC is obtained at pH≈8.75 and vaterite-like ACC precipitates from solutions of pH≈9.8 and higher. In addition to these two amorphous polymorphs, other studies have shown hints of aragonite local order in ACC from shells of freshwater snails, based on XAS data that show Ca-O coordination numbers of approximately 9, the theoretical value of aragonite. These results were reproduced in synthetic samples of ACC doped with Mg~(2+), suggesting a role of this cation in the selection of the ACC amorphous polymorph.