Electron Paramagnetic Resonance of V2+, Mn2+, Fe3+, and Optical Spectra of V3+ in Blue Zoisite, Ca2Al3Si3O12(OH)

Abstract

Electron paramagnetic resonance and optical absorption spectra have been used to clarify the local environments of transition metal ions and their distributions among various Al and Ca sites in a gem quality zoisite crystal from Tanzania. The EPR spectra due to Mn2+, Fe3+, and two types of V2+ have been interpreted by the spin Hamiltonian βH·g·S + I·A·S + D[Sz2 − 13S(S + 1)] + E(Sx2 − Sy2) in the principal axes system x, y, z. The Mn2+ ions occupy one of the Ca sites, probably Ca(1), with point group symmetry m; g = 2.003 ± 0.005 and | A | = (85 ± 2) × 10−4cm−1, both isotropic, and also | D | = (103 ± 5) × 10−4cm−1, | E | = (34 ± 2) × 10−4cm−1, in general agreement with other oxides and hydrates. Highly anisotropic hyperfine and small zero-field splittings (| D | < 0.014 cm−1) are observed for V2+, which is the opposite of the usual situation. One of the two types of V2+ apparently occupies the same site as Mn2+; the other V2+ ion is in a general position, and probably occupies a double minima potential around the Ca(2) site. The Fe3+ ions occupy the AlII site since the point group symmetry is m; the principal axes, however, are displaced approximately 45° from the NMR axes of 27Al. Apparently, the local environment changes when Al3+ ions are replaced by Fe3+. For Fe3+, we found | D | = 0.14 ± 0.03 cm−1 and | E | ∼ 0.1 | D |. The optical absorption of V3+ and the trichroism of the zoisite single crystal are consistent with the two types of Al3+ site symmetries, with crystal field parameters Dq = 1850 and 1400 cm−1 at AlI and AlII sites.