Structure and physical property relations of Mn ilvaite

Structural and compositional data as well as ⁵⁷Fe Mössbauer parameters were determined on a natural Mn-rich monoclinic ilvaite crystal (ideal composition CaFe ₂ ²⁺ Fe³⁺Si₂O₈(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 [001] 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 [001] 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 M1 sites could be estimated.