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Crystallographic and structural transformations of sedimentary chalcedony in flint upon heat treatment
Authors:Patrick Schmidt,Sylvie MasseGuillaume Laurent,Aneta SlodczykEric Le Bourhis,Christian PerrenoudJacques Livage,Franç  ois Frö  hlich
Affiliation:a Muséum national d’histoire naturelle, Departmento de Préhistoire UMR 7194, Centre de spectroscopie infrarouge, CP 57, 57, rue Cuvier 75231 Paris Cedex 05, France
b Chimie de la Matière Condensée de Paris, UPMC Univ Paris 06, CNRS, Collège de France, 11 place Marcelin Berthelot, 75005 Paris, France
c Laboratoire de dynamique, interactions et réactivité (LADIR) UMR 7075, 2 rue Henry Dunant 94320 Thiais, France
d Université de Poitiers, Département de physique et mécanique des matériaux, Institut P’ UPR 3346, Boulevard Marie et Pierre Curie, BP 30179, F86962 Futuroscope Chasseneuil Cedex, France
e Muséum national d’histoire naturelle, Departmento de Préhistoire UMR 7194, Centre européen de recherches préhistoriques, avenue Léon-Jean-Grégory, 66720 Tautavel, France
Abstract:The early occurrence of intentional heat treatment of silica rocks has recently become a key element in the discussion about the cultural modernity of prehistoric populations. Lithic vestiges are the only sources that remain of this process and the understanding of the material’s properties and transformations are essential for reconstructing the conditions and parameters applied during heat treatment. Several models of the structural transformations upon heating have been proposed in the current literature. These models are often contradictory and do not account for the most recent structural and mineralogical data on chalcedony. In order to propose a new model, we elaborated an experimental procedure and applied different techniques involving infrared spectroscopy, solid state NMR, X-ray diffraction and electron microscopy. The results show that the major transformation to happen is the loss of silanole (SiOH) and the creation of new Si-O-Si bonds according to the reaction: Si?OH HO-Si → Si-O-Si + H2O. This reaction starts between 200 °C and 300 °C and causes an increase in the hardness of the rocks. The maximal annealing temperature and the ramp rate are the functions of the ability of the structure to evacuate newly created H2O and depend on the size of the specimen and the volume of its porosity. These results also show that the annealing duration at maximum temperature can be relatively short (<50 min) for a sufficiently large amount of transformation to be accomplished.
Keywords:Length-fast chalcedony   Flint   Heat treatment   Annealing   Hydroxyl   Silanole   Porosity
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