The Influence of Silicate on the Transformation of Lepidocrocite to Goethite

U. Schwertmann* and R. M. Taylor
C.S.I.R.O., Division of Soils, P.O. Glen Osmond, South Australia 5064
* Present address: Institut für Bodenkunde Techn. Univers. München, 8050 Freising, West Germany. The research was carried out at the C.S.I.R.O., Division of Soils, Adelaide, South Australia whilst on a visiting fellowship.

Abstract: The mechanism of the transformation of lepidocrocite (γFeOOH) to goethite (αFeOOH) has previously been established and the effect of silicate on the transformation was investigated. Rather than completely inhibiting the reaction, as had been suggested, the presence of Si was found to merely retard the nucleation stage of the transformation. There was found to be no decrease in the dissolution rate of the lepidocrocite due to surface adsorption of Si.

Si has no effect if introduced after the nucleation stage, and under conditions of pH and temperature where the dissolution rate of the lepidocrocite largely determines the rate of transformation, the presence of Si has a reduced effect. The results show that Si is adsorbed and incorporated into the goethite structure, and due to its retarding effect on the nucleation, larger crystals of goethite are formed, many of which are twinned.

It is inferred from the results that the apparent stability of lepidocrocite occurring in soils in association with goethite cannot be attributed solely to the presence of Si in the soil system.

Clays and Clay Minerals; June 1972 v. 20; no. 3; p. 159-164; DOI: 10.1346/CCMN.1972.0200307
© 1972, The Clay Minerals Society
Clay Minerals Society (www.clays.org)