Publication Date:
2013-11-19
Description:
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 Fe 2+ and Mn 2+ 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, Mössbauer spectroscopy, optical absorption spectroscopy, bond-valence theory, ionic radius concept and literature, the occurrence of Fe 2+ at the Z site of tourmaline is well supported. Conversely, existing experimental data does not provide indisputable evidence for the occurrence of Mn 2+ at the Z site. Despite this, there is no evidence for inductive effects of Y Mn 2+ on 〈Z-O〉, and the proposed effects must be regarded as speculative. Statistical analysis shows that the 〈 Z Al-O〉 average value is 1.906(2) Å, which is consistent with the observed values of 〈 Z Al-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.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
Permalink