A spectroscopic study of Mn(II)-bearing calcites: comparison between inorganic calcite and biocalcite, obtained by the bacterium Bacillus subtilis

The first multi-analytical characterisation of a biogenic calcite, produced from bacterial activity, is reported in this study. Microcrystalline calcite powders were obtained from the model bacterium Bacillus subtilis grown on a suitable precipitation medium. The powders were analysed, without further manipulation, by means of X-ray powder diffraction (XRPD), and by Electron Paramagnetic Resonance (EPR) spectroscopy. Both techniques reveal unusual spectral parameters, attributed to the effects of the bioprecipitation of the mineral. In particular, XRPD pointed out an anomalous c0/a0 ratio, which was noticed before only in microcrystalline calcite samples grown by biomineralisation of seashells. The usual ratio for inorganic calcite is recovered by opportune thermal treatments. The specific anomalies in the parameters of the EPR spectroscopy mainly consists in an anomalously large distribution of the zero-field splitting interaction, which allows to clearly distinguish bacterial and inorganic calcite. These values, coupled to the data concerning the fine, hyperfine and Zeeman interactions, confirm the clear spectroscopic fingerprint of bacterial biocalcite. As a consequence, this study fosters the coupled use of XRPD and EPR for identifying the traces of bacterial activity in fossil carbonate deposits, with particular reference to the reconstruction of palaeoclimate and in geochemical cycles of pollutants.