Identification of iron-containing impurities in natural kaolinites using the Mössbauer effect

Abstract

SUMMARY. Two clay samples originating in the kaolinized granite area near St. Austell, Cornwall, have been examined by MSssbauer spectroscopy. The iron-containing impurity in the sample with a red discoloration (sample A) has, by measurement of the hyperfine magnetic field, been identified as hematite. In the case of sample B, which was yellow in colour, preliminary MSssbauer work indicated fi-FeOOH as the impurity, but detailed measurements of hyperfine field and N6el temperature, when compared with similar results for a synthetic/3-FeOOH sample, suggested otherwise. X-ray and chemical work suggested that the iron-containing phase in sample B is goethite, ~-FeOOH in which some substitution of Al+8 for Fe z+ has occurred; the MSssbauer data are consistent with this conclusion. Both clay samples exhibited superparamagnetic behaviour and this has been utilized to obtain rough estimates of the mean radius of the particles of the iron-containing impurity. These were 117/~ for the ~-Fe~O8 in sanaple A and 270/~ for aluminian c~-FeOOH in sample B.IT has been established by Malden and Meads (I967) by Mtissbauer exp ~'riments on a natural kaolinite purified by magnetic separation that Fe z+ substitutes, almost certainly in the octahedral cation site, in the kaolinite lattice. It is important to distinguish this situation from another that commonly exists, namely the occurrence of iron in separable impurities in the clay, usually in mica, or as oxides and oxy-hydroxides of iron. The experiments here described illustrate the usefulness of M6ssbauer work in the identification of impurities of the latter type. Some recent Mossbauer experiments by Janot et al. (I973) on kaolinites of tropical soil origin have yielded results that are generally similar to those reported here, but the measurements were restricted to two temperatures, 85 K and 3o0 K. As will be shown, more information regarding the iron-rich impurity phase can be obtained by experiments at a number of intermediate temperatures.The association of iron hydroxides with the surfaces of kaolinite particles has been studied by chemical, X-ray, and electron microscopic methods by Greenland and Oades (I968). Under certain circumstances the iron compound may be rather poorly crystallized.Sample description and preparation. Two samples of kaolinite from a clay pit in the St. Austell area of Cornwall were studied: both showed coloration, sample A having a red tint and sample B a yellow tint. Each clay contained about 2 % by 9

Mössbauer Study of the Attenuation of Iron in an Irrigated Greensand Lysimeter

“…On cooling a sample of the uncontaminated sediment the spectral area of the ferric doublet was decreased, and a magnetic component appeared that formed about 50% of the total area at 77~ and 4.2~ (Figure 2). The hyperfine field value at 4.2~ of 499 kOe is in better agreement with that expected for goethite, 506 kOe (van der Kraan and van Loef, 1966), than that for /3-FeOOH, 470 kOe (Hogg et al, 1975). Since no magnetic component in the room temperature spectrum was found, it is likely that the goethite is present as finely divided particles (Shinjo, 1966;van der Kraan and van Loef, 1966;Govaert et al, 1977), although the possibility that it contains some substituted aluminum cannot be ruled out on the basis of the field values (Bigham et al, 1978).…”

Hydrothermal (200°C) synthesis and crystal chemistry of iron-rich kaolinites

“…6). Unless the particle size of the iron oxides is extremely small, in which case they could be paramagnetic, the occurrence of well crystallized ferric components is also excluded (Janot et al, 1973;Hogg et al, 1975;Bonnin et al, 1982;Fysh et al, 1983).…”

“…Hogg et al (1975) noted the importance of distinguishing between kaolinite structural iron and iron in impurities, and fine particles of hematite and goethite were identified by them and Janot et al (1973). Jefferson et al (1975) found that the nonstructural iron was included in a non-crystalline gel, coating the surface of the kaolinite flakes, or in a fine coating of goethite.…”

Mössbauer Effect Studies of Iron in Kaolin. I. Structural Iron

“…The considerable number of studies employing ESR techniques have recently been reviewed by Hall (1980). Hogg et al (1975) noted the importance of distinguishing between Fe present within the kaolinite structure and Fe located in the impurities such as mica and iron oxides/oxyhydroxides which are common in kaolin ores. Both they and Janot et al (1973) identified fine particles of hematite and goethite in kaolin samples.…”