N. ChaikumR. M. CarrMahidol University2018-06-142018-06-141987-01-01Clay Minerals. Vol.22, No.3 (1987), 287-296000985582-s2.0-0023479587https://repository.li.mahidol.ac.th/handle/123456789/15335Absorption lines in the g approx 4 region of ESR spectra of three New Zealand halloysites were found to originate from similar paramagnetic centres to those in kaolinites. Resonances in the g approx 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 018O - 2 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-T lines exhibited by all samples are due to V 4+ or VO 2+ replacing Al 3+ in the octahedral sheet. -D.J.M.Mahidol UniversityEarth and Planetary SciencesArticleSCOPUS10.1180/claymin.1987.022.3.03