A hole-like centre present in natural hydroxylapophyllite without any artificial irradiation has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy at 290 K and 90 K, and by three-pulse electron spin echo envelope modulation (ESEEM) spectroscopy at 25 K. Calculated matrices g, A(29Si), A(1H), A(39K) and P(39K) suggest it to be an O− centre at the hydroxyl oxygen site and the unpaired electron in the 2Pz orbital. A series of weak satellite peaks accompanying the main absorption line in single-crystal EPR spectra have been shown to arise from four geometrically distinct pairs of neighbouring O− centres (i.e., biradicals). The spin Hamiltonian parameters of these O− −O− pairs provide further support for the O− model and its location at the hydroxyl oxygen site. The O− centre in hydroxylapophyllite most likely formed from natural radiation and can be enhanced by gamma-ray irradiation. It is annealed out at 300°C but can be restored readily by gamma-ray irradiation. The presence of these O− −O− pairs in the hydroxylapophyllite is probably attributable to a high abundance of the O− centre in this sample.

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