Thermal treatment of the tourmaline Fe-rich princivalleite Na(Mn 2 Al)Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 O

Natural Fe 2+ -rich princivalleite was thermally treated in the air at 700 °C to study crystal-chemical and color variations due to changes in oxidation states of Fe and Mn and atom ordering. Overall, the experimental data (electron microprobe, structural refinement, Mössbauer, infrared, and optical absorption spectroscopy) show that thermal treatment of princivalleite results in an almost total Fe 2+ oxidation to Fe 3+ and an oxidation of approximately one-third of Mn 2+ to Mn 3+ along with a minor degree of disorder of Al–Fe–Mn over the Y and Z sites. This process is accompanied by a significant deprotonation of the sample. The Y Fe and Y Mn oxidation from + 2 to + 3 yields in a decrease in a -parameter, whereas the increased content of Z Fe 3+ results in a minor increase in the c- parameter. Optical absorption spectroscopy shows that the faint blue (azure) color of untreated princivalleite is caused by the presence of Fe 2+ and the absence of Ti 4+ . Thermal treatment in air (700 °C) changed the color to dark brown due to the progressive oxidation of Fe 2+ to Fe 3+ and Mn 2+ to Mn 3+ , as demonstrated by the evolution of optical absorption bands caused by electron transitions in these 3 d -cations. However, the most evident result of the thermal treatment of the Fe-rich princivalleite sample is the simultaneous presence of Fe 2+ , Fe 3+ , Mn 2+ , and Mn 3+ , with a Fe 3+ /ΣFe and Mn 3+ /ΣMn ratio of 0.92 and 0.25, respectively. This observation suggests that the oxidation process during the heating experiments was largely controlled by kinetic factors.