Anisotropic peak broadening analysis of a biogenic soil greigite (Fe3S4) with Rietveld analysis and single peak fitting

A new approach for modeling anisotropic peak broadening with Rietveld analy sis has been tested on a biologically grown soil greigite, whose morphology (as determined by TEM, Stanjek et al. 1994) had a symmetry lower than cubi c. The anisotropic morphology of this greigite determined with single peak fitting was more evident than with the Rietveld analysis in the cubic setti ng, because in Rietveld analysis {hhh} reflections are treated as one refle ction, although two different peak breadths are required: one for modeling two (hhh) reflections perpendicular to the [111] axis of elongation and one for modeling the remaining six (hhh) reflections. For assessing the lower morphological symmetry, the cubic structure was transformed into the tetrag onal and into the trigonal system. In both crystal systems, the axis of sha pe anisotropy is [001]. Due to splitting of peaks, the anisotropic morpholo gy became then measurable. Contributions from other likely sources of aniso tropic peak broadening, such as stacking faults and chemical inhomogeneitie s could be ruled out. The cell-edge length a = 9.868(4) Angstrom agrees with previous values (Sta njek and Murad 1994). The isotropic temperature factors of B (Fe-8a) = 1.16 (6) Angstrom(2), B (Fe-16d) = 1.7(2) Angstrom(2) and B (S-32e) = 110(8) Ang strom(2) are much smaller than the overall factor B = 3.2(4) Angstrom(2) de termined by Skinner et al. (1964), whereas the anion positional parameter u = 0.2535(3) is identical with 0.2505(38).