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Manganese accommodation in fossilised mastodon ivory and heat-induced colour transformation

¹ Centre de Recherche et de Restauration des Musées de France - UMR 171 CNRS, Museé du Louvre, 6 rue des pyramides, F-75041 Paris cedex 01, France
² Laboratoire de Minéralogie Cristallographie, UA CNRS 7590, Universités Paris 6 et 7 - IPGP, case 115, 4 place Jussieu, F-75252 Paris cedex 05, France
³ European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble cedex, France
⁴ Laboratoire de Physique des Liquides et Electrochimie, UPR 15 CNRS, case 133, 4 place Jussieu, F-75252 Paris cedex 05, France

^* Corresponding author: present address: Rathgenforschungslabor Staatliche Museen zu Berlin, Stiftung PreuBischer Kulturbesitz, SchloBstr. 1a, D-14059 Berlin, Germany; e-mail: rf{at}smb.spk-berlin.de

Odontolite or bone turquoise, a turquoise-blue coloured heated fossilised mastodon ivory or bone, is an imitation of the mineral turquoise used for the decoration of medieval art objects. Its colour origin can be ascribed to Mn⁵⁺ traces in a tetrahedral environment of four oxygen atoms in an apatite matrix.

The structural environment of Mn was investigated by means of extended X-ray absorption fine structure (EXAFS) in order to shed new light on the colour transformation of fossilised ivory induced by a thermal treatment. This was particularly difficult as the manganese concentration does not exceed trace amounts in fossilised ivory. EXAFS experiments confirm the presence of Mn^2+/3+/4+ ions before and Mn⁵⁺ ions after thermal treatment in fossilised ivory and the location of Mn⁵⁺ ions on a P analogous site in apatite.

Key-words: odontolite, bone turquoise, apatite, colour origin, Mn..

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