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

Powder X-ray diffraction (XRD) curves were calculated for the differen t structural models so far proposed for feroxyhite (deltaFeOOH). The i nfluence on XRD features of different structural parameters, including site occupancy of Fe atoms, atomic coordinates, content and distribut ion of stacking faults, and dimension of coherent scattering domains, were considered. On the basis of agreement between experimental and si mulated curves it is shown that deltaFeOOH is a mixture of feroxyhite proper and ultradispersed hematite in the 9: 1 volume ratio. Feroxyhit e proper consists of hexagonal close packing of anions containing 5% s tacking faults. Iron atoms occupy only octahedral sites and are distri buted in such a way that face-sharing filled octahedral pairs regularl y alternate with vacant octahedral pairs along the c axis. This distri bution of Fe atoms is quite similar to that established by Patrat et a l. (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 o ctahedron. Nearest Fe-Fe distances calculated for the model proposed ( 2.88, 3.01, 3.39 and 3.73 angstrom) practically coincide with those fo und by EXAFS spectroscopy for the same sample (2-91, 3.04, 3.41 and 3. 7-3.8 angstrom).