Nickel site distribution and clustering in synthetic double-chain silicates by experimental and theoretical XANES spectroscopy

The crystal-chemical role of Ni in synthetic potassium-richterites within the join K(CaNa)Mg4Ni[Si8O22] (OH)(2)-K(CaNa)Ni-5[Si8O22] (OH)(2) has been investigated by XAS (x-ray-absorption spectroscopy). From a structural point of view these materials are classified as double-chain silicates and belong to the amphiboles group. X-ray absorption near-edge structure (XANES) spectra recorded at the Ni K edge show variations that can be related to the Ni-Mg substitution at the octahedral M sites of these amphiboles. Theoretical XANES spectra, calculated on the basis of the multiple-scattering theory, reproduce very well the experimental spectra, and allow us to understand the relationships between XANES features and the chemical and structural environments around the photoabsorbing ion. The combination of XAS experiments and theoretical calculations provides information on the Ni partitioning between the three octahedral sites and on the local chemistry around the absorber.