Abstract
A natural single crystal of amethyst was investigated by means of continuous-wave and pulsed Electron Paramagnetic Resonance (EPR), with the aim of structurally characterizing the substitutional S2 Fe(III):H+ centre. In this centre, Fe(III) replaces Si(IV) in the tetrahedral site, whereas H+ is coupled to Fe(III) to maintain the charge balance. The spectroscopic investigations, mainly the interpretation of the Electron Spin Echo Envelope Modulation, allowed a detailed localisation of the proton to be obtained. H+ occurs in the channels crossing the crystal parallel to the crystallographic c axis, in a largely eccentric position. The Fe(III)-H+ distance, evaluated in 2.70 Å, is found associated with a non-negligible isotropic hyperfine coupling, which can be linked to the relative stability of the S2 centre in natural amethyst.
Highlights
A natural single crystal of amethyst was investigated by means of continuous-wave and pulsed Electron Paramagnetic Resonance (EPR), with the aim of structurally characterizing the substitutional S2 Fe(III):H+ centre
Fe in quartz can occur in a variety of structural settings and chemical environments, being either interstitial or straight replacing Si in its tetrahedral site
When Fe(III) replaces Si(IV) in the tetrahedral site, a charge defect is added to the system
Summary
A natural single crystal of amethyst was investigated by means of continuous-wave and pulsed Electron Paramagnetic Resonance (EPR), with the aim of structurally characterizing the substitutional S2 Fe(III):H+ centre In this centre, Fe(III) replaces Si(IV) in the tetrahedral site, whereas H+ is coupled to Fe(III) to maintain the charge balance. Extensive continuous wave EPR (cwEPR) characterisations of Fe-bearing quartz single crystals, where the presence of the S2 centre has been assessed, are reported in [5], for natural samples, and in [2, 4] for synthetic samples These studies enlighted the occurrence of two S2 centres, one more abundant (the socalled S2(D), with the proton connected to the distant oxygen) than the other (the S2(C) centre, with the proton linked to the close oxygen). In a study by [10], carried out on the so-called S0 defect, i.e. the [FeO4/h]0 centre, while accounting for the former studies on synthetic samples, and for the spectroscopic results therein included, a simplified Hamiltonian operator was used to interpret a continuous wave
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