Heat induced transformation of fossil mastodon ivory into turquoise 'odontolite'. Structural and elemental characterisation

摘要

The present work deals with the structural and elemental analysis of turquoise mineral imitations as 'odontolite' or bone turquoise by transmission electron microscopy (TEM), scanning electron microscopy (SEM-EDX) and particle induced X-ray and #gamma#-ray emission (PIXE-PIGE). The aim of the work is to evidence the former deliberate transformation of fossilised ivory by man in order to transform them into semi-precious stones. We show that the crystal structure of 'odontolite' artefacts consisting of fluorapatite (Ca_5(PO_4)_3F) corresponds to that of heated fossil mastodon ivory (12-15 million years old). Metallic traces detected by PIXE-PIGE in these 'odontolites' are discussed in order to explain their role for coloration. Other more greenish turquoise imitations have a bone-like structure and composition, and carbonate-by-droxylapatite. The presence of copper salts at the surface is responsible for their green coloration.