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Electron paramagnetic resonance spectroscopy of Fe3+ ions in amethyst: thermodynamic potentials and magnetic susceptibility

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Abstract

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe3+ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E 1′. However, the two broad signals do not correspond to any known Fe3+ centers in the quartz lattice, but are most likely attributable to Fe3+ clusters on surfaces. The absolute numbers of spins of the Fe3+ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy E a, entropy of activation ΔS and enthalpy of activation ΔH for the Fe3+ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe3+ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.

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Acknowledgments

We thank Dr Milan Rieder and the two anonymous referees for incisive criticisms and helpful suggestions, Dr KV Narasimhulu and Prof. JL Rao for helpful discussions, Dr Satya Singh for provision of CuSO4·5H2O, and the Natural Science and Engineering Research Council (NSERC) of Canada for financial support.

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  1. Gobburu SivaRamaiah

    Present address: Department of Physics, Government College for Men, Kadapa, 516004, India

Authors and Affiliations

  1. Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, S7N 5E2, Canada

    Gobburu SivaRamaiah, Jinru Lin & Yuanming Pan

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  1. Gobburu SivaRamaiah
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Correspondence to Yuanming Pan.

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SivaRamaiah, G., Lin, J. & Pan, Y. Electron paramagnetic resonance spectroscopy of Fe3+ ions in amethyst: thermodynamic potentials and magnetic susceptibility. Phys Chem Minerals 38, 159–167 (2011). https://doi.org/10.1007/s00269-010-0391-2

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