Origin and Position of Exchange Sites in Kaolinite: An ESR Study

Electron spin resonance (ESR) spectroscopy is used to investigate the nature of exchange sites on kaolinite. ESR spectra of exchangeable Cu 2+ and Mn 2+ on kaolinite indicate that divalent exchange ions are about 11–12Å apart on kaolinite surfaces and that planar Cu (H 2 O) 4 2+ ions are oriented parallel to the surfaces. Solution-like spectra for exchangeable Cu 2+ and Mn 2+ are observed at high relative humidities, suggesting a high degree of mobility of exchange cations on kaolinite surfaces. The evidence seems to eliminate edge sites as being active in cation exchange at least in the acidic range of pH. Similar conclusions are derived from ESR studies of Cu 2+ -saturated talc and pyrophyllite. It is proposed that most exchange sites arise from ionic substitutions or mineral impurities in phyllosilicates.