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.
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