Alterations of fossil-bearing shale (Autun, France; Permian), part III: Framboidal pyrite and sulfur as the main cause of efflorescence

摘要: Fossil-bearing shale specimens that include sulfides are chemically reactive and sometimes also mechanically fragile. This decay is provoked by iron sulfate efflorescence resulting from the oxidation of sulfide compounds. The processes underlying these degradations poorly known, thus impeding elaboration curative or preventive treatments. present contribution aims to identify origin museum specimen alterations. It focuses on Flouest collection housed at Museum national d'Histoire naturelle (MNHN, Paris, France) originating Autun Basin (Saone-et-Loire, France; Permian). To evaluate alteration MNHN specimens, it appeared necessary compare their composition with unaltered so as chemical changes occurring during ageing. Therefore new material was collected in Basin, among others locality Muse corresponds same lithostratigraphic unit than specimens. work divided three parts. two first, presented elsewhere, deal matrices led conclusion could not account for large iron(II) provoking damage last part this work, here, focus artificial age-ing experiments performed material. Most alterations observed artificially aged samples correspond dispersed crystals calcium (gypsum). Similar may be found but they relatively few sporadically distributed. They considered secondary importance respect efflorescence. These latter reproduced only (upon 142 samples). were a copro-lite wood remain had got severely damaged ambient conditions within months after excavation. On all grown brownish layers consisting framboidal pyrite (1 3 m) eventually sulfur (20 50 m). associated thin maceral probably because bacterial activity: fossil diagenesis, bacteria need organic matter metabolism produce hydrogen sulfide, precursor sedi-mentary formation. show well layer nearby one them, particularly thick. vitrinite shows some areas aspect. topology surface (observed scanning electron microscopy) numerous small holes (< 2 (10–20 morphology compatible former occurrence isolated grains aggregates pyrite. observations suggest