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Gouzy, S., Rondeau, B., Gaudin, P., Louarn, G., La, C., Lebeau, T., Vinogradoff, V., Clodore, L. & Chamard-Bois, S. (2023) Whitening of fire opal: Transformation of silica in soils. Chemical Geology, 616 121237.
Added by: Richard Baschera (2023-01-20 09:53:33) Last edited by: Richard Baschera (2023-01-20 09:56:59) |
| Type de référence: Article DOI: 10.1016/j.chemgeo.2022.121237 Numéro d'identification (ISBN etc.): 0009-2541 Clé BibTeX: Gouzy2023 Voir tous les détails bibliographiques  |
Catégories: MIOPS Créateurs: Chamard-Bois, Clodore, Gaudin, Gouzy, La, Lebeau, Louarn, Rondeau, Vinogradoff Collection: Chemical Geology |
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Of all the world's gem opal deposits, Ethiopian's ones stand out for their quantitative production of white opals and orange fire opals. They also provide a significantly amount of intriguing specimens displaying a translucent, porous, white periphery covering a transparent, massive, orange core. These ``zoned opals{''} are stable (they do not evolve with time after mining) and are only encountered in Ethiopia thus far. This suggest that the envi-ronment of formation and residence of opals, attributed to pedogenetic processes in a strongly weathered environment, play an important role in the formation of those specimens. We hypothesized that ``zoned opals{''} are a transitional state between orange and white opal due to a progressive and centripetal transformation caused by fluids in the soil. First, we document the properties of zoned opals for the first time and compare them to entirely orange opals from Ethiopia (Wegel Tena and Mezezo) and Mexico, by multi-scale techniques: UV-visible absorption spectrometry, Raman spectroscopy, LA-ICP-MS, Atomic Force Microscopy and Scanning Electron Microscopy. White periphery proved to be more porous, depleted in iron, and structurally more polymerized compared to the orange cores. Second, we conduct experimental modeling of the ``external whitening{''} phe-nomenon as it could occur in a pedogenetic environment rich in both organic and inorganic fluids. For that, we submitted orange opals to various solutions of reactants naturally occurring in soils or acting as good proxies for acidolysis and/or complexolysis: oxalic acid, sulfuric acid, EDTA, pyoverdine and deferoxamine. Some of organic and inorganic solutions at pH = 1, proved to satisfactorily model the natural whitening. As for natural opals, whitened samples were more porous and depleted in iron. However, the polymerization state of the silica network significantly decreased. Moreover, through the short time scale of our experiments, it evolved in a non-linear manner. That is why, depolymerization and increase of porosity may induce repolymerization with longer times after the external whitening destabilization phenomenon, as it is seen for 30-20 million years old naturally whitened samples.
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