Heat-induced color changes of fossilized Miocene mastodon ivory (13–16 Ma) have been known at least since the Middle Ages. Cistercian monks are believed to have created odontolite, a turquoise-blue “gemstone,” by heating mastodon ivory found in Miocene geological layers next to the Pyrrenean chain, France, to use it for the decoration of medieval art objects. This material has been the object of investigations of famous European naturalists and gemmologists, among them Réaumur (1683–1757). Although vivianite [Fe3(PO4)2·8H2O] is the commonly accepted coloring phase supposed to appear when heating fossilized mastodon ivory, our previous spectroscopic studies using PIXE/PIGE and TEM-EDX demonstrated that the chemical composition of collection odontolite and heated mastodon ivory corresponds to well-crystallized fluorapatite [Ca5(PO4)3F] containing trace amounts of Fe (230–890 ppm), Mn (220–650 ppm), Ba (160–620 ppm), Pb (40–140 ppm), and U (80–210 ppm). No vivianite has been detected.
To provide new insights into the physico-chemical mechanism of the color transformation of fossilized ivory, we used the combination of UV/visible/near-IR reflectance spectroscopy, time-resolved laser-induced luminescence spectroscopy (TRLIF), and X-ray absorption near-edge structure (XANES).
Contrary to what had formerly been described as the color origin in odontolite, our study has conclusively identified traces of Mn5+ by UV/visible/near-IR reflectance spectroscopy, TRLIF, and XANES inside the fluorapatite. Thus, odontolite owes its turquoise-blue color to Mn5+ ions in a distorted tetrahedral environment of four O2− ions. XANES also demonstrated oxidation of disordered octahedral Mn2+ ions to tetrahedral Mn5+ species in apatite during the heat process. So we give the first evidence of the real color origin in odontolite.