Istituto di Geoscienze e Georisorse     
Di Benedetto F., D'Acapito F., Fornaciai G., Innocenti M., Montegrossi G., Pardi L., Tesi S., Romanelli M. A Fe K-edge XAS study of amethyst. In: Physics and Chemistry of Minerals, vol. 37 (5) pp. 283 - 289. Springer, 2010.
An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO2) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe2+ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell: = 1.78(2) , the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe3+ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe4+ is stabilised by the favourable local structural arrangement.
URL: http://https://link.springer.com/journal/269
Subject Amethyst
DFT calculations
Fe bioavailability

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