Clay Minerals (1991) 26, 233-244 A POTASSIUM-RICH BEIDELLITE FROM A LATERITE PALLID ZONE IN WESTERN AUSTRALIA BALWANT SINGH AND R. J. GILKES Soil Science and Plant Nutrition, The Universityof Western Australia, Nedlands, WA 6009, Australia (Received 14 May 1990; revised 13 September 1990) A B S T R A C T: A smectite formedby weathering of mica in a laterite pallid zone at Boddington, Western Australia has been investigatedby analytical electron microscopy.The evidencesuggests that this mineral has the chemical composition and swelling properties of ideal beidellite but is anomalousin containingabundant non-exchangeable K. This K balanceshalfof the layerchargethat arises mostlyfrom tetrahedral substitution of A13+for Si 4+, with the K+ occupying one sixth of the sites occupied by K in mica. Smectite which resembles beidellite may be a weathering product of dioctahedral mica (Ross & Mortland, 1966; Egashira & Tsuda, 1983). Beidellite in soils is characterized by the swelling of a Mg-saturated sample treated with glycerol and the irreversible collapse to 10 of a K-saturated air-dried sample. Minerals of this type occur in various soil environments e.g. lignite overburdens in Texas (Senkayi et al. 1983; Egashira et al. 1982), weathering gneiss in Poland (Stoch & Sikora, 1976), and weathered granitic rocks at Kyushu, Japan (Egashira & Tsuda, 1983). This paper describes the properties of a beidellite-like clay mineral that may have formed by weathering of mica in a lateritic pallid zone in SW Australia, and compares the properties of this mineral to those of reference smectites. MATERIALS Boddington sample #1 and sample #2 These samples were supplied by Hedges Gold Pty. Ltd., Australia. They had been obtained from drill holes into a deeply weathered laterite profile developed from a micaceous granite at the Hedges Gold Mine at Boddington 120 km SSE of Perth, Western Australia. Samples were taken from the lateritic pallid zone at depths of 300 cm and 340 cm below the surface (sample #1 and sample #2 respectively). The general nature of the laterite pallid zone in this region has been described by McCrea et al. (1990) with feldspars and micas being replaced by pseudomorphs consisting dominantly of kaolinite and/or halloysite. Reference smectites Black Jack Mine (BJM) beidellite (#NMNH 24679) was obtained from the Smithsonian Institution, Washington. Unterrupsroth beidellite (B 17) was kindly supplied by Prof. G. 9 1991 The MineralogicalSociety