STRUCTURAL MODEL FOR FERRIHYDRITE

The structure of 6-line and 2-line ferrihydrite (Fh) has been reconsid ered. X-ray diffraction (XRD) curves were first simulated for the diff erent structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the b asis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh con sisting of anionic ABACA ... close packing in which Fe atoms occupy on ly octahedral sites with 50% probability; the hexagonal unit-cell para meters are a = 2.96 angstrom and c = 9.40 angstrom, and the space grou p is P31cBAR. (ii) Defective Fh in which Ac1Bc2A and Ab1Cb2A structura l fragments occur with equal probability and alternate completely at r andom; Fe atoms within each of these fragments have identical ordered distribution within the hexagonal super-cell with a = 5.126 angstrom. (iii) Ultradispersed hematite with mean dimension of coherent scatteri ng domains (CSD) of 10-20 angstrom. The main structural difference bet ween 6-line and 2-line Fh is the size of their CSD which is extremely small for the latter structure. Nearest Fe-Fe distances calculated for this new structural model are very close to those determined by EXAFS spectroscopy on the same samples.