STRUCTURE OF FEROXYHITE AS DETERMINED BY SIMULATION OF X-RAY-DIFFRACTION CURVES

Powder X-ray diffraction (XRD) curves were calculated for the different structural models so far proposed for feroxyhite (deltaFeOOH). The influence on XRD features of different structural parameters, including site occupancy of Fe atoms, atomic coordinates, content and distribution of stacking faults, and dimension of coherent scattering domains, were considered. On the basis of agreement between experimental and simulated curves it is shown that deltaFeOOH is a mixture of feroxyhite proper and ultradispersed hematite in the 9: 1 volume ratio. Feroxyhite proper consists of hexagonal close packing of anions containing 5% stacking faults. Iron atoms occupy only octahedral sites and are distributed in such a way that face-sharing filled octahedral pairs regularly alternate with vacant octahedral pairs along the c axis. This distribution of Fe atoms is quite similar to that established by Patrat et al. (1983), but in each pair, Fe atoms are displaced by the same value of 0.3 angstrom in opposite directions away from the centre of their octahedron. Nearest Fe-Fe distances calculated for the model proposed (2.88, 3.01, 3.39 and 3.73 angstrom) practically coincide with those found by EXAFS spectroscopy for the same sample (2-91, 3.04, 3.41 and 3.7-3.8 angstrom).