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SINGLE-CRYSTAL EPR STUDY OF Fe³⁺ AND VO²⁺ IN PREHNITE FROM THE JEFFREY MINE, ASBESTOS, QUEBEC

Yuanming Pan, Mao Mao and Jinru Lin

Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada

E-mail address: yuanming.pan{at}usask.ca

Prehnite from the Jeffrey mine, in Asbestos, Quebec, has been investigated by optical microscopy, electron-microprobe analysis, single-crystal X-ray-diffraction (XRD) analysis, and electron paramagnetic resonance (EPR) spectroscopy. Electron-microprobe analyses yield an average iron content of 0.053(34) wt% Fe₂O₃; single-crystal XRD intensity datasets confirm the space group P2cm. Single-crystal EPR spectra measured from 290 to 13 K reveal at least six paramagnetic defects: three Fe³⁺ centers, a VO²⁺ radical, a Mn²⁺ center, and an Al-associated O^– center. Spin-Hamiltonian parameters of the VO²⁺ radical show that it resides at an octahedral site and most likely originated from a V³⁺O₄(OH)₂ octahedron by removal of a proton and trapping of a hole to form a covalent V=O bond during natural radiation. Spin-Hamiltonian parameters, including the high-spin terms of type S⁴, BS³ and BS⁵, for the dominant Fe³⁺ center also show it to reside at an octahedral site. Two less abundant Fe³⁺ centers have similar orientations in effective principal axes of g, indicative of occupancies at the octahedral site as well. Their differences in effective principal values of g may be attributed to different degrees of local distortion of the octahedral site. A previous interpretation of the powder EPR spectra for two Fe³⁺ sites in prehnite has been shown to be incorrect. Spin-Hamiltonian parameters for Fe³⁺ in prehnite have wide implications for interpretation of EPR spectra of Fe³⁺ in layer silicates.

Keywords: prehnite, single-crystal EPR, VO²⁺, ferric iron, high-spin terms, site occupancy, local distortion, layer silicates, Jeffrey mine, Quebec.