Abstract

An Fe-bearing chromo-alumino-povondraite sample from the Sludyanka crystalline complex (Lake Baikal, Russia) was characterized using single-crystal X-ray diffraction, electron microprobe, Mössbauer, infrared, and optical absorption spectroscopy. The symmetry is rhombohedral, space group R3m, with unit-cell parameters a = 16.0032(2), c = 7.2823(1) Å, V = 1615.15(4) Å3, Z = 3. The crystal structure of the Fe-bearing chromo-alumino-povondraite was refined to an R1 index for all reflections of 1.74% using MoKα X-ray intensity data. Crystal chemical analysis resulted in the empirical structural formula:  
(Na0.76Ca0.190.03K0.02)X(Cr1.943+Mg0.93Al0.07Ti0.064+)Y(Al3.74Mg1.43Fe0.323+V0.193+Cr0.163+Fe0.152+)Z[(Si5.99Al0.01)TO18](BO3)B3(OH)V3[O0.69F0.23(OH)0.08]W.

This formula, in which Cr prefers the Y site and Al the Z site, is compatible with the end-member NaCr3(Al4Mg2)(Si6O18)(BO3)3(OH)3O. Magnesium is disordered over Y and Z, whereas Fe and V3+ are ordered at Z.

Ideally, chromo-alumino-povondraite is related to oxy-chromium-dravite and oxy-dravite by the homovalent substitution Cr3+ ↔ Al3+. Tourmaline chemical compositions classified as chromo-alumino-povondraite can be either Cr-dominant or Al-dominant as a result of the compositional boundaries along the solid solution between Cr and Al that are determined at Y+Z(Cr5Al2), corresponding to NaY(Cr3)Z(Cr2Al2Mg2)Si6O18(BO3)3(OH)3O, and Y+Z(Cr1.5Al5.5), corresponding to NaY(Cr1.5Al1.5)Z(Al4Mg2)Si6O18(BO3)3(OH)3O.

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