Mossbauer Spectra of Soil Kaolins from South-western Australia

-Six well-characterized soil kaolins from widely separated sites in south-western Australia and four reference kaolins were studied by 57Fe M6ssbauer spectroscopy at room temperature after removal of non-structurally-bound iron with dithionite-citrate-bicarbonate solution. The soil kaolins and one of the reference kaolins were also studied at temperatures near 16 K. The soil kaolins were remarkably similar in crystal size, crystaUinity, dehydroxylation temperature, cation exchange capacity, surface area and iron content. M6ssbauer spectra of the soft kaolins at room temperature were also essentially identical consisting of a quadrupole-split doublet superimposed on a broad component which indicated that all of the iron was present as Fe(III) and that slow paramagnetic relaxation effects were present. Mean values for the chemical isomer shift and quadrupole splitting of the doublet for the soil kaolins were 0.33 and 0.55 mm/s respectively which indicates that the iron is in the octahedral sites of the kaolin lattice. The spectra of the soil kaolin samples at temperatures near 16 K showed a further slowing down of the paramagnetic relaxation and confirmed that no discrete iron oxide minerals were present. M6ssbauer spectra of the four reference kaolins at room temperature showed a doublet component similar to those for the soil kaolins. Three of them showed evidence for other spectral components including, in two cases, a component due to the presence of Fe(II). Key Words--Iron, Kaolin, M6ssbauer spectroscopy. I N T R O D U C T I O N Kaolin is the most abundant and widespread clay mineral in deeply weathered soils in south-western Australia (Singh, 1991). The crystallinity of soil kaolin varies considerably but kaolins from highly weathered soils of tropical and sub-tropical regions are usually disordered and of sub-micron size (Hughes and Brown, 1979; Dixon, 1989; Singh and Gilkes, 1992). The presence o f iron in the kaolin structure contributes towards the poor crystallinity o f the mineral (Herbillon et aL, 1976; Mestdagh et aL, 1980). Several techniques including bulk chemical analysis, infrared spectroscopy, electron paramagnetic resonance, M6ssbauer spectroscopy, electron microprobe analysis, and single crystal analytical electron microscopy indicate the presence of iron in the kaolin structure (Dixon, 1989; Singh and Gilkes, 1992). Since soil kaolin generally occurs in a mixture of minerals, some of which contain iron, it is difficult to assign the iron to particular minerals on the basis of the above techniques. M6ssbauer spectroscopy has been successfully used to determine the presence of iron in kaolin in the absence of other iron-containing minerals (Malden and Meads, 1967; Fysh et aL, 1983; Murad and Wagner, 1991). It is therefore likely to prove of value in the investigation of forms of iron in purified soil kaolin especially since the signal produced is specific to the iron in a sample. 3 Person to whom correspondence should be addressed. Copyright 9 1992, The Clay Minerals Society The aim of this study of soil kaolins was to determine: (1) whether iron was within the kaolin structure or in other minerals, e.g., iron oxides; (2) the oxidation state of the iron; (3) whether structurally bound iron was in the octahedral or tetrahedral sites of kaolin. The results of M6ssbauer spectroscopy are also compared with Fe contents determined by bulk chemical analysis using X-ray fluorescence (XRF) and single crystal analysis using analytical electron microscopy (AEM).