Crystallographic and spectroscopic characterization of Fe-bearing chromo-alumino-povondraite and its relations with oxy-chromium-dravite and oxy-dravite

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: X(Na0.76□ 0.03K0.02)Y(Cr1.94 3+Mg0.93Al0.07Ti0.06 4+)Z(Al3.74Mg1.43Fe 0.32 3+V0.19 3+Cr0.16 3+Fe0.15 2+) [T(Si 5.99Al0.01O18)O18] B(BO3)3 V(OH)3 W[O0.69F0.23(OH)0.08]. This formula, in which Cr prefers the Y site and Al the Z site, is compatible with the end-member NaCr3(Al4Mg2)(Si6O 18)(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+ ↔ Al 3+. 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(Cr2Al2Mg 2)Si6O18(BO3)3(OH) 3O, and Y+Z(Cr1.5Al5.5), corresponding to NaY(Cr1.5Al1.5) Z(Al4Mg2)Si6O18(BO 3)3(OH)3O.