On the role of Fe2+ in the luminescence of feldspar: an exploration of Fe2+/Fe3+ L-edge near edge x-ray absorption fine structure using comparative electronic and optical detection methods

The luminescence properties of Fe3+ defects in feldspar are well know, but the role played by Fe2+ centres remains ambiguous, with tentative previous evidence suggesting it could act as a luminescence activator, luminescence quencher, or neither one nor the other: the outcome of these arguments would be of direct and topical relevance to consideration of using Fe3+ luminescence for the in situ dating of Martian sediments in future probe missions to the planet. Therefore, to elucidate the relative roles of Fe2+ and Fe3+, a synchrotron radiation study of Fe2+/Fe3+ defects in feldspar has been undertaken by probing the L-edges of the ion. Partial electron yield results allow a measure of the relative proportion of the ion states to be made, but of itself provides no indication of the defect's optical activity: in order to determine the latter, optical detection of x-ray absorption spectroscopy has been deployed. Although minor differences in the Fe2+/Fe3+ ratios are established using the two measurement methods, the results of the survey indicate that Fe2+ is unlikely to be an efficient luminescence quenching centre, and that competition for charge between the ion in its 2+/3+ states is not radically different. Equally, no evidence was found to suggest that it efficiently contributes to the visible emission in the range 380-750 nm. (paper)