The conversion of montmorillonite to interstratified halloysite-smectite by weathering in the Omi acid clay deposit, Japan

Abstract The weathering of montmorillonite in the acid clay deposit at Itoigawa, Japan has been investigated by X-ray diffraction (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). In the lower, least weathered part of the deposit, weathering has occurred by the formation of minor amounts of halloysite with concomitant precipitation of a silica phase identified as opal C-T. A relatively rapid transition occurs to the most weathered, upper part of the deposit where the mineralogy is dominated by interstratified halloysite-smectite. Computer simulations of XRD profiles indicate that the interstratified arrangement is random with a probable composition of 70% halloysite and 30% smectite, a composition supported by the 75–85% and 15–25% of the phases, respectively, calculated from the IR spectra.