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On the presence of hydrous defects in differently coloured wulfenites (PbMoO4): an infrared and optical spectroscopic study |
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| Authors: | D Talla M Wildner A Beran R ?koda Z Losos |
| Institution: | 1. Institut für Mineralogie und Kristallographie, Universit?t Wien, Althanstra?e 14, 1090, Wien, Austria 2. Department of Geological Sciences, Masaryk University, Kotlá?ská 2, 611 37, Brno, Czech Republic 3. CEITEC, Masaryk University, Kotlá?ská 2, 611 37, Brno, Czech Republic |
| Abstract: | Several samples of wulfenite, PbMoO4, varying in colour from colourless to yellow, orange and red, have been characterised by means of IR and optical absorption spectroscopy and by microprobe analyses. A distinct pleochroic band group with absorption maxima centred at 3,380 and 3,150 cm?1 can be seen in the IR spectra of wulfenite single-crystals, indicating the presence of hydroxyl groups. The pleochroic and thermal behaviour of the OH stretching bands along with deuteration experiments, as well as results obtained from synthetic flux-grown samples, exclude the presence of submicroscopic hydrous mineral inclusions as their primary origin. The pleochroic scheme and the band positions were used to postulate a model for the OH incorporation mode, based on the assumption of vacancies on Mo and Pb sites in the structure of this ‘nominally anhydrous mineral’. Optical absorption spectra of coloured natural samples show a broad and polarised band around 23,000–24,000 cm?1, preceding the fundamental UV absorption edge, which has been identified as the reason for the colour of the mineral. The comparison with synthetic PbMoO4 single-crystals, doped with variable amounts of Cr6+, yielded conclusive evidence that trace amounts of the CrO4 2? anion group, substituting for MoO4 2?, determine the variable colour. Besides, in one sample, trace amounts of Nd3+ have been spectroscopically identified. |
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