A critical comment on Ertl et al. (2012): “Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: Evidence from Fe2+- and Mn2+-rich tourmaline”.

In this paper we have presented a detailed response to Ertl et al. (2012a) who, in a paper in volume 97 (year 2012), pages 1402-1416, this journal, claim evidence for limitations of Fe2+ and Mn2+ occupancy at the Z site of the tourmaline structure. They also propose a model by which the <Z-O> distance of tourmaline varies as a function of its <Y-O> and <T-O> bond lengths. We have examined their conclusions and find that a different distribution of cations over the Y and Z sites gives better agreement with the extensive experimental information available. In fact, on the basis of crystal-structure refinements, Mossbauer spectroscopy, optical absorption spectroscopy, bond-valence theory, ionic radius concept and literature, the occurrence of Fe2+ at the Z site of tourmaline is well supported. Conversely, existing experimental data does not provide indisputable evidence for the occurrence of Mn2+ at the Z site. Despite this, there is no evidence for inductive effects of Mn-Y(2+) on <Z-O>, and the proposed effects must be regarded as speculative. Statistical analysis shows that the <Al-Z-O> average value is 1.906(2) angstrom, which is consistent with the observed values of <Al-Z-O> at the 99% confidence limit (within 3) in tourmalines with the Z site fully occupied by Al. Consequently, the proposed inductive effect of <Y-O> and <T-O> on <Z-O> can be ruled out.