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 t he join K(CaNa)Mg4Ni[Si8O22] (OH)(2)-K(CaNa)Ni-5[Si8O22] (OH)(2) has been i nvestigated by XAS (x-ray-absorption spectroscopy). From a structural point of view these materials are classified as double-chain silicates and belon g to the amphiboles group. X-ray absorption near-edge structure (XANES) spe ctra recorded at the Ni K edge show variations that can be related to the N i-Mg substitution at the octahedral M sites of these amphiboles. Theoretica l XANES spectra, calculated on the basis of the multiple-scattering theory, reproduce very well the experimental spectra, and allow us to understand t he relationships between XANES features and the chemical and structural env ironments around the photoabsorbing ion. The combination of XAS experiments and theoretical calculations provides information on the Ni partitioning b etween the three octahedral sites and on the local chemistry around the abs orber.