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 degrees 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, Mossbauer, 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 degrees 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 Fe-Y + (Z)(Mg+Al) Fe-Z + (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.