A Mossbauer study of the role of iron impurities in the high temperature reactions of kaolinite minerals

Abstract

Mossbauer spectroscopy was used to study the valence and bonding changes of iron in (a) lattice sites, and (b) cation exchange sites during thermal treatment of kaolinite and halloysite. Lattice iron which is initially in octahedral sites occurs in octahedral and tetrahedral sites at the completion of the thermal reactions. At temperatures below 650°C some divalent ions in cation exchange positions become trivalent and assume octahedral co-ordination, which they retain until the later stages of mullite formation. The implications of the work on the high temperature reaction mechanism and structural sequence are discussed.

Mössbauer Study of Transformations Induced in Clay by Firing

“…Thus the asymmetry in the coordination of iron is expected to increase, leading to an increase in quadrupole splitting, while the increased disorder would give rise to inhomogeneous broadening due to variations of the quadrupole splitting from site to site. Similar results have been found by Mackenzie (1969) in a study of iron impurities in kaolinite and by Janot and Delcroix (1974) who have studied the effects of firing on a typical clay in order to draw conclusions about the firing temperatures of ancient French ceramics. On the other hand the dehydroxylation process in this range of temperatures has been studied extensively by DTA techniques.…”

Effect of structural iron on nanoscroll formation via exfoliation of a high iron-content kaolin

“…Iron substitution in clays is considered to follow a random distribution [16], and while Fe 3+ and rarely Fe 2+ can be found in the O-sheet, only Fe 3+ is reported in the T-sheet [17][18][19]. Fe 3+ generally retains the six-fold coordination [20], while Fe 2+ can be present in six-or four-fold coordinations [21,22] as well. There are a few suitable analytical methods for the investigation of structural iron in clays, Mössbauer spectroscopy [23], X-ray absorption spectroscopy [24], and the slightly less frequently applied X-ray photoelectron spectroscopy [25], being the most often reported.…”