• journal_title：American Mineralogist
• Contributor：Ferdinando Bosi ; Henrik Skogby ; Ulf Hålenius ; Leonid Reznitskii
• Publisher：Mineralogical Society of America
• Date：2013-08-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4447
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：8-9
• firstpage：1557
• section：Articles

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) ų, 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:

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 Na^Y(Cr₃)^Z(Cr₂Al₂Mg₂)Si₆O₁₈(BO₃)₃(OH)₃O, and ^Y+Z(Cr_1.5Al_5.5), corresponding to Na^Y(Cr_1.5Al_1.5)^Z(Al₄Mg₂)Si₆O₁₈(BO₃)₃(OH)₃O.