Absorption lines in the g ∼ 4 region of ESR spectra of three New Zealand halloysites were found to originate from similar paramagnetic centres as in kaolinites. Resonances in the g ∼ 4 region were not common to all the halloysites, although the signal at g = 2·0 for the Matauri Bay halloysite is probably due to a trapped positive hole or trapped ion. A 6-line resonance produced by Te Puke halloysite and removable by Na-saturation is attributed to exchangeable Mn on the clay surface. Low-temperature lines exhibited by all samples are due to V4+ or VO2+ replacing Al3+ in the octahedral sheet.
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Chaikum et al. (1987) studied this question.