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Structural Model for Ferrihydrite

Published online by Cambridge University Press:  09 July 2018

V. A. Drits ,
B. A. Sakharov ,
A. L. Salyn  and
A. Manceau
V. A. Drits
Affiliation:
Geological Institute of the Russian Academy of Science, 7 Pyzhevsky prospekt, 109017 Moscow, Russia, and
B. A. Sakharov
Affiliation:
Geological Institute of the Russian Academy of Science, 7 Pyzhevsky prospekt, 109017 Moscow, Russia, and
A. L. Salyn
Affiliation:
Geological Institute of the Russian Academy of Science, 7 Pyzhevsky prospekt, 109017 Moscow, Russia, and
A. Manceau
Affiliation:
LGIT-IRIGM, Université Joseph Fourier and CNRS, BP53X, 38041 Grenoble, France
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Abstract

The structure of 6-line and 2-line ferrihydrite (Fh) has been reconsidered. X-ray diffraction (XRD) curves were first simulated for the different structural models so far proposed, and it is shown that neither of these corresponds to the actual structure of ferrihydrite. On the basis of agreement between experimental and simulated XRD curves it is shown that Fh is a mixture of three components: (i) Defect-free Fh consisting of anionic ABACA . . . close packing in which Fe atoms occupy only octahedral sites with 50% probability; the hexagonal unit-cell parameters are a = 2-96 Å and c = 9-40 Å, and the space group is P1c. (ii) Defective Fh in which Ac1Bc2A and Ab1Cb2A structural fragments occur with equal probability and alternate completely at random; Fe atoms within each of these fragments have identical ordered distribution with in the hexagonal super-cell with a = 5.26 Å. (iii) Ultradispersed hematite with mean dimension of coherent scattering domains (CSD) of 10-20 Å. The main structural difference between 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.

Type
Research Article
Information
Clay Minerals , Volume 28 , Issue 2 , June 1993 , pp. 185 - 207
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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