E in Halloysite : Insight from Room - Temperature Experiments

Halloysite is a common pedogenic clay mineral, often found in young soils developed on volcanic deposits (Dixon, 1989). It is a member of the kaolin group of clays with the same ideal stoichiometric composition as kaolinite [A12Si2Os(OH)4 ]. Halloysite, however, often contains water of hydrat ion (i.e., A12Si2Os(OH)4.nH20), and is commonly found with a tubular morphology. This "rol l ing" of halloysite has received a great deal of study because there is no generally agreed upon mechanism for the process and there is no corresponding phenomenon in natural kaolinite (e.g., Bates et aL, 1950; Bailey, 1989; Singh, 1996; Singh and Mackinnon, 1996). The crystal structure of halloysite often shows stacking disorder. This property, combined with a rolled morphology, makes identification by X-ray diffraction (XRD) difficult. The XRD peaks at 7.5, 4.4, and 3.6 ,~ are often asymmetric with a large width at half peak height (Bailey, 1989). Oxygeniso tope systematics of hal loysi te are thought to be similar to kaolinite (Savin and Epstein, 1970), but a recent review suggested a small fractionation (--1%o) between the two minerals (Sheppard and Gilg, 1996). Hydrogen-isotope systematics of halloysite have not been investigated in great detail, but prel iminary measurements (Lawrence and Taylor, 1972) suggested that H in halloysite is easily exchangeable at room temperature. Hsieh (1997) used selective dissolution, centrifugation, and XRD techniques to extract and identify pedogenic halloysite from some Hawaiian soils for measurement of ~180 values. These isotopic analyses produced low oxygen yields and ~ 8 0 values which differed from those expected for a mineral with isotopic characteristics similar to kaolinite. Small amounts of two of these purified halloysite samples remained from the work of Hsieh. We report the results of hydrogen isotope exchange experiments