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If we detect changes in the chemical composition (mostly zonal development or the occurrence of different mineral phases) via electron microprobe, it is also possible to assume the existence of structural changes visible in the Raman spectra in form of a band shifts or bandwidth variations. Presence of such changes may relate to substitution mechanisms and distribution of different impurities may be confirmed by LA-ICP-MS 2D imaging (especially sub-ppm elements previously not detected on EPMA). Interrelationships between structure and chemical composition can then be sought in interesting correlations between these techniques.
Rare hyalite opal showing green daylight fluorescence was discovered in the Erongo region, Namibia, sometime prior to 2000. Raman spectroscopy confirms this hyalite to be opal-A, and the most interesting feature of the Raman spectra are bands associated with the hydroxyl-stretching regions in the mineral's structure. A doublet at 3602/3665 cm-1 appears to be unique to opal from this locality and may be due to the vibration of water in an unusual structural-binding arrangement. Trace amounts of the uranyl molecule (UO 2) 2+ are responsible for the luminescence, which is unevenly distributed according to the opal's botryoidal structure. Measured radioactivity values are similar to those reported for other daylight-fluorescing opals (e.g. from Mexico), and are well below the global average terrestrial gamma dose rate.
