Rozhdestvenskaya, I. V.; Vereshchagin, O. S.; Frank-Kamenetskaya, O. V.; Zolotarev, A. A.; Pekov, I. V. (2011): About crystal-chemical formula of chromdravite - mineral species from the tourmaline group. Zapiski Rossiiskogo Mineralogicheskogo Obshchestva (Proceedings of the Russian Mineralogical Society) 140, 93-99 (in Russian).
The crystal structure of tourmaline with high chromium content (~32 wt % of Cr2O3) from micaceous metasomatites of Onega flexure (a = 16.154, c = 7.427 Å, sp. gr. R3m, R = 0.040, Rw = 0.043) has been refined. The disorder distribution of Cr3+ and Mg2+ cations over Y- and Z-octahedral sites described by the ideal formula Na(Cr2Mg)(Cr4Mg2)(BO3)3(Si6O18)(OH)4 was revealed. The proximity of the common element composition of tourmaline examined to the composition of original chromdravite from the same deposit necessitates the revision of the ideal formula of this mineral species approved before by the commission of IMA.
Edited 2 time(s). Last edit at 01/21/2012 04:31PM by Uwe Kolitsch.
An interesting consequence of this disorder is that the distinction between chromium-dravite and hypothetical "chromium-schorl" is lost, since both have Cr3+ as the dominant cation on both types of octahedral site.