Dedicated to Prof. Dr U. Schwertmann on the occasion of his 75 th birthday Iron in soil kaolins from Indonesia and Western Australia R. D. HART 1, *, T. G. ST PIERRE 2 , R. J. GILKES 1 , A. J. MCKINLEY 3 , S. SIRADZ 1 AND BALWANT SINGH 1, { 1 Department of Soil Science and Plant Nutrition, University of WesternAustralia, Perth, W.A. 6009, 2 Department of Physics, University of Western Australia, Perth, W.A. 6009, 3 Department of Chemistry, University of Western Australia, Perth, W.A. 6009, Australia (Received 15 April 2001; revised 20 July 2002) ABSTRACT: Soil kaolins from Indonesia and Western Australia and a range of reference kaolins were studied using Mo ¨ssbauer spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry. Mo ¨ssbauer spectra indicate that the Fe within the kaolins is in the high- spin Fe 3+ oxidation state and that a large fraction of the Fe is present as dispersed atoms residing within the octahedral sites of the kaolinite crystal structure. The EPR spectra are typical for soil kaolins except for the absence of radiation-induced defects for the Indonesian kaolins. The Fe (I) spectra are dominant with a strong symmetric peak at g = 4.3, the presence of Fe (II) spectra is shown by a shoulder on this peak at g = 4.9 and a small phase up peak at g = 9.2. Low-temperature (5 K) magnetization (M) measurements over large field (H) sweeps (Ô70 kOe) yielded M(H) curves which are fitted well with Brillouin functions indicating the paramagnetic nature of the kaolins at temperatures down to 5 K. A very small remanent magnetization was detectable in the kaolins. Remanent magnetization to saturation magnetization ratios ranged from 10 4 to 10 3 for the Indonesian kaolins and were all ~10 3 for the Western Australian kaolins, indicating that at high fields the vast majority of the magnetization of the kaolins is due to paramagnetic ions. Zero-field-cooled and field-cooled magnetization measurements in small fields (500 Oe) indicate that the Indonesian kaolins are generally free from magnetically-blocked material down to a temperature of 5 K. The magnetic susceptibility of the Indonesian kaolins shows Curie Law behaviour indicating paramagnetic behaviour over all temperatures down to 5 K. Measurements on the Western Australian kaolins indicated the presence of some magnetic material that is magnetically blocked at temperatures below ~200250 K. As a consequence, the magnetic susceptibility showed large deviations from Curie Law behaviour. KEYWORDS: iron, soil, kaolin, Indonesia, Australia, Mo¨ssbauer spectroscopy, electron paramagnetic resonance, SQUID magnetometry. The structural, chemical and surface chemical properties of soil kaolins are remarkably different from those of standard mineral kaolins. Soil kaolins incorporate higher concentrations of Fe 3+ cations than mineral kaolins (up to 4% Fe 2 O 3 ) and Fe 3+ is known to influence properties such as crystal size and order/disorder of the structure, and thus also affects properties such as cation and anion * E-mail: r.d.hart@exchange.curtin.edu.au DOI: 10.1180/0009855023740069 { Present address: Department of Agricultural Chemistry and Soil Science, University of Sydney, Australia ClayMinerals (2002) 37,671–685 # 2002 The Mineralogical Society