Arsenic incorporation in natural calcite: a structural and spectroscopic study.

An experimental and theoretical study was undertaken on crystal chemical features of the C↔As substitution in natural calcite lattice, which is suggested to occur in As-bearing travertine rocks from Tuscany (Italy). This study has been carried out by means of As K-edge XAFS (X-ray Absorption Fine Structure), Electron Paramagnetic Resonance, Electron Spin Echo spectroscopy, Density Functional Theory (DFT) calculation and X-ray diffraction. Samples are constituted by calcite associated to minor amounts of silicates (mainly quartz and phyllosilicates) and Fe-oxyhydroxides. Travertine rocks formed during Quaternary in a paleo lake. Here both abiotic and biotic precipitation of calcite were possible.XAS experiment and DFT calculation indicate that As is incorporated in the crystal lattice of calcite. The formal valence of As is As(III) indicating that CO32- ↔ AsO33- substitution is feasible. X-ray diffraction and crystal parameter refinement point out to an appreciable expansion of the c axis of As-bearing calcite. XAS experiment, in addition, suggest that the C sites replaced by the CO32- ↔ AsO33- mechanism are not homogenously distributed in the lattice but are rather clustered around Mn(II) ions substituting for Ca(II) in the octahedral sites. This conclusion is supported by the presence of strong signal (i.e. with coordination number close to the maximum allowed for the calcite cell) from an As-As coordination shell to the EXAFS signal of a travertine sample and by ESE spectroscopy.