In vitro synthesis and structural characterization of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) was prepared it? vitro by different methods. The preparation from carbon dioxide and aqueous calcium hydroxide gave a fully X-ray amorphous phase without any additive. The precipitation from calcium chloride and sodium carbonate in the presence of either magnesium or ethylene-diamine-N,N,N',N'-tetrakis-methylene-phosphonic acid (EDTMP) also gave fully amorphous solids. All ACC phases contained about 7 wt% of residual water and transformed into crystalline phases upon contact with gaseous or liquid water. These materials are model compounds for biogenic ACC which is of importance in biomineralisation either as functional material or as a transient phase on the way to calcite or aragonite. If ACC is prepared it? vitro without any additive, a clear short-range order around calcium can be determined by X-ray absorption spectroscopy (EXAFS). Additive-free ACC converts exclusively to crystalline calcite upon heating or in contact with water and may therefore act as a model compound for biologically found transient precursor phases.