Extended X-ray absorption fine structure (EXAFS) studies on calcium in crystalline and amorphous solids of biological interest

Using synchroton radiation, the calcium K-edge extended x-ray absorption fine structure (EXAFS) for a number of calcium-containing solids of biological interest was measured. These included synthetic analogues of platelet dense bodies, rat tibia mineral, and synthetic bone-like calcium phosphates. The mean CaO distances in all of the solids examined were essentially the same. However, in contrast to the crystalline calcium solids studied, the dense body analogues showed sufficient structural disorder that CaP and CaCa atom pair contributions to the EXAFS spectra could not be observed. The absence of these second-shell contributions suggest that these analogues are truly amorphous rather than microcrystalline in structure. The EXAFS spectrum of the bone mineral was more similar to carbonate-apatite than to any of the other synthetic apatites studied.