Thermally induced cation redistribution in fluor-elbaite and Fe-bearing tourmalines

An Fe-rich fluor-elbaite was thermally treated in air and hydrogen atmosphere up to 800 °C to study potential changes in Fe- and Al-ordering over the octahedrally coordinated Y and Z sites. Overall, the experimental data (structural refinement, electron and ion microprobe, Mössbauer, infrared and optical absorption spectroscopy) show that thermal treatment of fluor-elbaite results in an increase of Fe contents at the Z site balanced by an increase of Al at the Y site. On the basis of this and previous experimental studies on Fe–Mg–Al-bearing tourmalines, it can be stated that the intersite Fe–Mg–Al exchange rates are significant at temperatures around 700–800 °C. Thermal treatment results in an increase of ca. 0.30 Fe atoms per formula unit at the Z site compensated by a similar increase of (Mg + Al) at the Y site, following the exchange reaction Y Fe + Z (Mg + Al) → Z Fe + Y (Mg + Al). Since the tourmaline nomenclature is based on the occupancy of ions at each structural site, the intersite Fe–Mg–Al ordering may determine the tourmaline species. This means that effectively the name associated with a given composition may be a function of the sample thermal history.