Mn crystal chemistry in pumpellyite: A resonant scattering powder diffraction Rietveld study using synchrotron radiation

The present study attempts to clarify the structural role of Mn in two pumpellyite specimens with the chemical formulas Ca-7.80(Al8.91Mn1.85Mg1.16Fe0.37)Si11.84O56-xOHx and Ca-8.12(Al7.99Mn3.41Mg0.71Fe0.30)Si11.84O56-xOHx. The samples are from gabbroic breccias of the Bracco ophiolites (Eastern Ligurian Apennines, Italy), and their chemistry is typical of Mn-rich pumpellyite minerals associated with Mn mineralization in prehnite-pumpellyite metamorphic facies. The distribution of the Mn cations over the two crystallographically independent X and Y octahedral sites, and their prevalent oxidation states, were determined by Rietveld refinement from multiple powder data sets collected at different wavelengths for each sample using synchrotron X-rays. The powder data, collected in proximity to and far from the MnK absorption edge, allowed refinement of the f' anomalous scattering parameter for each data set along with other variables during the Rietveld structure analysis, and we could thus estimate the oxidation state of Mn from the valence chemical shift of the absorption edge. The results indicate that Mn in pumpellyite is distributed over both octahedral sites, and the refined anomalous scattering coefficients clearly show that the two Mn valence states are segregated: Mn2+ is prevalent in the more symmetrical octahedral X site, and Mn3+ is prevalent in the octahedral Y site. The present results confirm the relationship between site partitioning and oxidation state of transition elements in pumpellyite that was previously proposed to explain the distribution of Fe cations in the structure.