Depolarisation currents (TSDC) and paramagnetic resonance (EPR) of iron in amethyst

Abstract

Correlated Thermally Stimulated Depolarisation Currents (TSDC) and Electron Paramagnetic Resonance (EPR) studies were performed on natural samples of Brazilian amethyst. Four bands of dipole origin are identified in the TSDC spectra at 185, 200, 214 and 260 K, respectively, where the three former originate a broad band centred at 215 K. The X-band EPR spectrum, for the magnetic field aligned parallel to the x-axis of the crystalline field of a given [FeO 4] − tetrahedron, is consistent with the spin Hamiltonian parameters D=(−8.630±0.001) GHz and E=(1.610±0.001) GHz. The EPR of the powder sample shows a broad band, centred at g ef=2.0, of line width of about 0.7 kG between the maximum slopes. The line intensity decreased monotonically under thermal treatments, for temperatures varying between 373 and 773 K, following a similar behaviour as observed using the TSDC technique. Substitutional and interstitial Fe 3+ ions provide the dominant features of the EPR spectra of amethyst, where Fe 3+ replaces Si 4+ in three equivalent sites of the structure. The similar behaviour of the TSDC and EPR isothermal decays suggests that the dipole centres are formed by a substitutional Fe 3+ and an O − or an OH − ion occupying a vertex of the [FeO 4] − tetrahedron. The differences in the intensity and in the position of the bands are a consequence of the unequal distribution of substitutional Fe 3+ centres among the three crystallographycally Si 4+ equivalent sites.