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A Fe K-edge XAS study of amethyst

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Abstract

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: <Fe–O> = 1.78(2) Å, 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.

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Acknowledgments

The authors acknowledge the Tuscany Administration for funding this research under the programme “Progetto di ricerca per l’individuazione delle cause di variazione della reattività superficiale della silice cristallina, nei principali comparti di lavoro toscani, in relazione alla sua potenziale patogenicità”. Italian CNR is also acknowledged for support. Authors acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities during experiments SI1593 and SI1773. Authors are also indebted to L. Pellicci of the Ce.Ri.Col Lab. for the ICP-AES investigations, and to N. Capolupo and P. A. Pozzi of the University of Florence and G. Saviozzi for sample preparation. F. dA. acknowledges E. Gliozzo for kindly permitting the publication of the data relative to the glassy sample. The manuscript benefited of the stimulating review by Y. Pan and an anonymous reviewer to whom authors express their warmest thanks.

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Authors and Affiliations

  1. Department of Chemistry, University of Florence, Florence, Italy

    Francesco Di Benedetto, Massimo Innocenti, Silvia Tesi & Maurizio Romanelli

  2. CNR-INFM-OGG c/o ESRF, Grenoble, France

    Francesco D’Acapito

  3. Department of Florence, ARPAT, Florence, Italy

    Gabriele Fornaciai

  4. IGG, CNR, Florence, Italy

    Giordano Montegrossi

  5. IPCF, CNR, Pisa, Italy

    Luca A. Pardi

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  1. Francesco Di Benedetto
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  2. Francesco D’Acapito
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Correspondence to Francesco Di Benedetto.

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Di Benedetto, F., D’Acapito, F., Fornaciai, G. et al. A Fe K-edge XAS study of amethyst. Phys Chem Minerals 37, 283–289 (2010). https://doi.org/10.1007/s00269-009-0332-0

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  • DOI: https://doi.org/10.1007/s00269-009-0332-0

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