Variations in the chemical composition of povondraite

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Geography and Geology


Povondraite is the most iron-rich tourmaline known, and has the end-member formula NaFe (super 3+) (sub 3) (Fe (super 3+) (sub 4) Mg (sub 2) ) (Si (sub 6) O (sub 18) ) (BO (sub 3) ) (sub 3) (OH) (sub 3) F. Although several researchers have reported tourmalines with a significant povondraite component, the species itself is quite rare. It often occurs as a small part of a larger crystal that is commonly zoned from dravite or "oxy-dravite" toward povondraite. We have examined numerous povondraite crystals from the type locality in Bolivia by EMPA and SREF. Povondraite has the highest reported values of K for any tourmaline: 2.39 wt%, 0.635 apfu (Zacek et al, 2000). The X site is also unlike many other tourmalines in that there seems to be very little vacancy. The chemical compositions are well-behaved with analytical totals that approach 100 wt% for ideal amounts of B (sub 2) O (sub 3) and H (sub 2) O, but normalization by standard methods results in X-site sums greater than 1 apfu. As Zacek et al. (2000) report similar results, any error would likely be due to an unforeseen difficulty in analyzing this particular material. We have examined a number of potential problems, including the possibility of overlapping peaks, interstitial K, mobilization of alkalis during analysis, and missing elements. The impossibly high values of alkalis reported for povondraite cannot easily be explained by errors in chemical analysis or missing constituents, and further work needs to be done on this issue. Fe (super 3+) in the tourmaline structure has a negative correlation with Al and a positive correlation with K. We suggest that the large amount of Fe (super 3+) in povondraite effectively expands the structure, allowing K to substitute for Na. This mechanism is supported by the fact that povondraite has a significantly larger unit cell than all other tourmaline species. Y and Z site-occupancies were determined by site-scattering and bond-lengths; these data show that the Z site is filled in the order Al, Mg, Fe (super 3+) . Residual divalent and trivalent cations are located at the Y site.