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Hyperspectral- and Thermo-Raman study of ornamental variscite

AutorPrado-Herrero, Pedro; García-Guinea, Javier ; Crespo Feo, Elena ; Correcher, Virgilio; Rubio, Juan
Fecha de publicación15-sep-2009
Citación5 th International Congress on the Application of Raman Spectroscopy in Art and Archaeology. Bilbao (Spain)14 th -18 th September 2009.
ResumenIn this study, we have analyzed a natural variscite for determining the distribution of Cr3+ and V3+ in the mineral structure by means of hyperspectral Raman. In the same context, a thermo-Raman study has been performed for controlling the mineral stability since hydrous phases tend to be transformed at temperatures as low as circa 60 ºC. The sample, chemically analyzed by EPMA, gives rise a composition of P2O5 45,98%, Al2O3 33,97%, H2O 19,56, Cr2O3 0.15% and traces elements (F, Ca, Zn, S, K and Sr). The observed spectra agree with the metavariscite features having the bands situated below 700 cm-1 corresponding to the lattice modes (PO4 out-of-plane bends and in-plane bends) and (PO4)3- antisymmetric vibration bands located at 1017 and 1077 cm-1. The presence of water in the metavariscite formula gives bands at the second order Raman spectrum due to the OH stretching vibrations (3119, 3369, and 3443 cm-1) as well as 1643 and 711 cm-1 peaks corresponding to HOH bending and water vibration, respectively. Generally, pseudo-lattice vibrations associated with aluminums can be seen in the region of 740-570 cm-1 for the anhydrous phase, then the alternated variations in the bands intensity in bellow 700 cm-1 area can be related to alterations in the Al3+ bonds. It seems that Cr3+ and V3+ substituted Al3+ are distributed as an alternation of Cr or V-rich clusters in the variscite structure. We recorded Thermo-Raman spectra from room temperature up to 500ºC. In this case, the most important change in the curves can also be observed in the temperature interval 150 ¿ 300 ºC. With the increasing temperature, H2O related bands become weaker, disappearing completely at 300ºC. It is worth to observe the alteration of bands 548, 578, 1017, and 1077 cm-1, which also disappear at 300ºC due to the meta-variscite transformation to berlinite. The bending peak placed at ~397 cm-1 loses intensity after 150ºC. After 300ºC, most intense Raman vibration bands for the new formed mineral were observed at 1138 and 469 cm-1.
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