Clay Minerals (1995) 30, 89-98 CHARACTERISTICS OF FINE PORES IN SOME HALLOYSITES G. J. CHURCHMAN, T. J. DAVY*, L. A. G. AYLMORE*, R. J. GILKES* AND P. G. SELF CSIRO Division of Soils, Glen Osmond, South Australia and *Soil Science and Plant Nutrition, Faculty oJA griculture, The University of Western Australia, Nedlands, Western Australia (Received 22 July 1993; revised 8 September 1994) ABSTRACT: Isotherms were obtained for nitrogen adsorption and desorption on seven halloysite- rich samples from New Zealand and Western Australia. Calculations from these isotherms indicate that halloysites with mainly small particles (< c. 0.08 I~m in width) had abundant cylindrical pores with narrow size distributions in the 5-15 nm range. They also indicate that halloysites with mainly large particles (> c. 0.1 ~tm in width) had few if any pores in the mesopore range (2-50 nm). Transmission electron microscopy (TEM) shows that cylindrical pores originate from the central holes in tubular particles. The TEM also suggests that slit-shaped pores can originate from the shrinkage of blocks of layers upon dehydration of halloysite. Halloysites include a significant concentration of fine pores (Diamond, 1970; Jackson et al., 1971; Churchman & Payne, 1983; McCrea & Gilkes, 1987; McCrea et al., 1990). Diameters recorded for the largest concentration of pores have depended upon the resolution of the techniques used for the measurement. Diamond (1970), Churchman & Payne (1983), McCrea & Gilkes (1987) and McCrea et al. (1990), all using mercury intrusion porisimetry, found that pores in the < 50 nm range provided most of the pore volume. Jackson et al. (1971), using isotherms for nitrogen and also for water sorption, found a sharp peak at 2.3 nm in the pore-size distribution. The high concentration of very small pores in halloysites contrasts with the situation in kaolinites. These tend to show larger pores without a marked concentration in any particular size range (Aylmore & Quirk, 1967; Diamond, 1970; Sills et al., 1973a; 1974; Aylmore & Sills, 1978). Generally, kaolinites comprise platy particles and pores in clay samples with platy particles are likely to form from the interleaving of the plates and should be similar in size to the thickness of the particles in the samples (Aylmore & Quirk, 1960; I967; Sills et al., 1973a) A review of the literature (Churchman & Carr, 1975) revealed that halloysites can be distinguished from kaolinites most reliably by the presence of interlayer water or by evidence for its prior occurrence in the structure. Halloysite particles often occur as non-platy shapes, and a tubular shape is common. The concentration of fine pores observed for halloysites has often been attributed to the tubular shape of many halloysite particles. The tubes have been considered to provide the characteristic small pores, both directly, from their hollow interiors, and also indirectly, from the voids created when they pack together with other tubes (Churchman & Payne, 1983; McCrea & Gilkes, 1987). Halloysite particles, however, can occur in shapes other than tubes, e.g. spheroidal and blocky shapes (Churchman & Carr, 1975; Churchman & Theng, 1984; Dixon, 1989). Until now, each study of pore-size distribution in halloysite has been carried out on only one sample of the mineral (Diamond, 1970; Jackson et al., 1971), on a single halloysite-containing soil (Churchman & Payne, 1983), or, at most, on a small number of halloysite-containing soils, but all from the same locality (McCrea & Gilkes, 1987; McCrea et al., I990). Particle shapes provided the explanations both for the apparently characteristic pore-size distribu- 1995 The Mineralogical Society