Alteration of fossil-bearing shale (Autun, France; Permian), part II: Monitoring artificial and natural ageing by combined use of S and Ca K-edge XANES analysis, Rock-Eval pyrolysis and FTIR analysis - Université de Bourgogne Accéder directement au contenu
Article Dans Une Revue Annales de Paléontologie Année : 2015

Alteration of fossil-bearing shale (Autun, France; Permian), part II: Monitoring artificial and natural ageing by combined use of S and Ca K-edge XANES analysis, Rock-Eval pyrolysis and FTIR analysis

Dégradation de schistes argileux fossilifères (Bassin d’Autun, France, Permien), partie II : vieillissements naturels et artificiels suivis par spectrométrie d’absorption au seuil de la raie K du soufre et du calcium, par pyrolyse Rock-Eval, et analyse IRTF

Résumé

Fossil-bearing shale specimens that include sulfides in their compositions are chemically reactive andsometimes also mechanically fragile. This decay is often related to iron sulfate efflorescence resulting fromthe oxidation of sulfide compounds. The processes underlying these degradations are poorly known, thusimpeding the elaboration of curative or preventive treatments. The present contribution aims to identifythe origin of museum specimen alterations. It focuses on the Flouest collection housed at the MuséumNational d’Histoire Naturelle (MNHN, Paris, France) and originating from the Autun Basin (Saône-et-Loire,France, Permian). To evaluate the alteration of MNHN specimens, it appeared necessary to compare theircomposition with that of unaltered shale so as to identify chemical changes occurring during ageing.Therefore, new material was collected in the Autun Basin, among others on the locality of Muse thatcorresponds to the same lithostratigraphic unit as that of the MNHN specimens. This material was, ifnecessary, artificially aged. The first part of this work, presented elsewhere, deals with the use of X-ray diffraction and Mössbauer spectroscopy for characterizing iron reactivity and speciation. It leads tothe conclusion that the reactivity of iron in the shale matrix was limited and could not account for thelarge efflorescence of iron (II) sulfate occurring nearby the fossil. The second part presented here focuseson the use of S K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy for characterizingsulfur speciation and reactivity. Measurements were performed on the shale matrix and on thin layersof maceral found in the proximity of damaged areas. As sulfur may be found in association with calciumor organic matter, complementary techniques were implemented, such as FTIR spectroscopy, Rock-Eval pyrolysis (characterization of organic matter content) and Ca K-edge XANES (analysis of calcium speci-ation) spectroscopy. It was shown that sulfur is mainly related to thioether, sulfoxide, iron sulfide, andsulfates whereas calcium is mainly bound to carboxylate, carbonate and/or sulfate groups. FTIR analy-sis of the macerals confirmed the presence of vitrinite on damaged MNHN specimens. The low oxygencontent of new shale samples determined by Rock-Eval pyrolysis indicates that the organic matter iswell preserved, despite the fact that samples come from outcrop surface. In the newly collected material,sulfur is mainly related to organic sulfides (thioether) with a minor occurrence of iron sulfide. In theshale fraction of damaged MNHN specimens, sulfur is mostly oxidized into a mixture of iron and calciumsulfate. However, in the vitrinite layers of the same specimens, a large proportion of sulfur correspondsto organic sulfides. Also the oxidation of sulfur does not occur homogeneously but preferentially in theshale fraction, probably because this latter is porous whereas vitrinite is not. Artificial ageing of newshale material showed that the oxidation of organic sulfides could be reproduced at 90◦C, 80% of rel-ative humidity. However, the obtained efflorescence almost exclusively corresponds to calcium sulfatewhereas iron (II) sulfates are mostly observed on MNHN specimens. The new material collected on siteis probably to be questioned, and future studies will have to select new samples with fossil remains. Thiswill be the object of the third part of this work.

Domaines

Paléontologie
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hal-01433294 , version 1 (12-01-2017)

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Giliane P. Odin, Frederik Vanmeert, François Farges, Georges Gand, Koen Janssens, et al.. Alteration of fossil-bearing shale (Autun, France; Permian), part II: Monitoring artificial and natural ageing by combined use of S and Ca K-edge XANES analysis, Rock-Eval pyrolysis and FTIR analysis. Annales de Paléontologie, 2015, 101 (3), pp.225-239. ⟨10.1016/j.annpal.2015.03.001⟩. ⟨hal-01433294⟩
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