Manganese accommodation in fossilised mastodon ivory and heat-induced colour transformation: Evidence by EXAFS

摘要

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 Mn~(5+) 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 Mn~(5+) ions after thermal treatment in fossilised ivory and the location of Mn~(5+) ions on a P analogous site in apatite.