Fehr, K. T.; Schneider, J.; Hochleitner, Rupert; Schmidbauer, E.
Structure and physical property relations of Mn ilvaite Part 1: Compositional structural and Mössbauer data.
In: Physics and Chemistry of Minerals, Vol. 32, Nr. 5-6: S. 388-399
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Structural and compositional data as well as 57Fe Mössbauer parameters were determined on a natural Mn-rich monoclinic ilvaite crystal (ideal composition CaFe22+Fe3+Si2 O8(OH)) which was used for electrical conductivity and thermopower measurements (part 2 of this paper). A zonar structure was found by electron microprobe analysis with a strong decrease in Mn concentration from the rim to the centre of the crystal in a plane perpendicular to the  direction. X-ray powder diffraction analysis of the most Mn-rich composition was performed. Mn2+ cations populate preferentially M2 sites of the ilvaite unit cell (space group P21/a), to a lower extent they reside on M1 and a reduced part is on Ca sites. The monoclinic angle was determined to β=90.178(4)°. The structural results are compared to literature data for other natural Mn-rich as well as low-impurity ilvaites; this concerns in particular the lattice b parameter and the undecided issue of the varying β angle. In the literature, the order parameter σ, which describes the varying degree of ordering of Fe2+-Fe3+ pairs on M11 and M12 sites in chains running parallel to the  direction, and structural defects are thought to be related to β. The interrelationship between β and σ with respect to a possible twin domain structure is discussed. Various 57Fe Mössbauer spectra were recorded between 151 K and 327 K. Mössbauer parameters and Fe2+/Fe3+ concentration ratios were determined from the fits to the spectra. Fitting of subspectra was accomplished with the idea to find assignments of Fe2+ and Fe3+ doublets in agreement with X-ray results. The fraction of Mn2+ substituting Fe2+ on M 1 sites could be estimated.