• journal_title：European Journal of Mineralogy
• Contributor：Mao Mao ; Mark J. Nilges ; Yuanming Pan
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2009/0021-1974
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：22
• issue：1
• firstpage：89
• section：Articles

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 2P_z 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.