Prehnite from Jeffrey Mine: structural study and chemistry

Referenza:
• Pan, Y., Mao, M., Lin, J. (2009): Single-crystal EPR study of Fe3+ and VO2+ in prehnite from the Jeffrey Mine, Asbestos, Québec. Canadian Mineralogist, 47, 933-945.

Abstract:
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.