Study of the influence of water and oxygen on the morphology and chemistry of pyritized lignite: Implications for the development of a preventive drying protocol

Abstract Lignite constitutes a unique testimony of past diversity and evolution of land plants. This material, usually waterlogged, is particularly difficult to dry because of its mechanical sensitivity to moisture changes. In addition, lignite may contain organic and inorganic sulfides, which are susceptible to oxidation once excavated. As a result, the conservation of lignite is particularly complicated and lignite remains scarce in paleobotanical collections. We experimentally test different drying protocols on waterlogged pyritized lignite, while documenting the respective role of water and oxygen on their morphology and chemistry. The results reveal that inorganic sulfides (pyrite) are more prone to oxidation than organic sulfides (thioethers). Critically, water is the main factor responsible for this oxidation, provoking sulfate efflorescence when samples are further exposed to oxygen. On the other hand, an abrupt removal of water provokes significant mechanical damage while sulfur remains mostly present as reduced compounds. The control of water and oxygen exchanges is thus critical for conserving lignite without physical damage and efflorescence. We successfully achieved this by storing the samples in hermetically sealed plastic bags made of semi-permeable films, which slowly release humidity while allowing a gradual influx of oxygen. We advise curators to quickly handle lignite once removed from its waterlogged environment because of the fast kinetics of oxidation, and to choose a drying protocol according to the purpose of the lignite treated. Finally, once dried, we advise to store the lignite in an anhydrous environment.