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
A study of natural oxy-tourmalines belonging to the system oxy-dravitechromo-alumino-povondraiteoxy-chromium-dravite from the Sludyanka crystalline complex (Southern Baikal region, Russia) was carried out to explore the characteristic vibrational bands in the principal (OH)-stretching frequency and their relations to the O3 Revised Manuscript Click here to download Manuscript: Cr-tm_IR ms O3_R1.docx Click here to view linked References 2 anion site of the tourmaline structure. Relevant information was obtained using electron microprobe analysis (EMPA), structural refinement (SREF), infrared (IR) and Raman single crystal spectroscopy. The studied oxy-tourmalines are characterized by the substitution Al ↔ Cr which is accompanied by redistribution of Mg over the Y and Z sites. The occurrence of strong correlations between relative peak area intensities for two IR bands at 3565 and 3519 cm-1 and cation site populations derived from SREF and EMP data allowed assignment of the band at 3565 cm-1 to the cluster [ Y Mg Z Al Z (Al,Mg)]-O3 and the band at 3519 cm-1 to the cluster [ Y Cr Z (Cr,Al) Z (Cr,Al,Mg))]-O3. It appears that the combination of polarized IR and Raman spectra collected with the electric vector E c and E // c may provide a useful characterization of the local (OH) environments around the O3 site of the tourmaline structure.
Recently it has been suggested that the major influence on the environment from Siberian Traps magmatism was due to the interaction of magma and organic-rich shale and petroleum-bearing evaporites, with the subsequent creation and outburst of toxic gases (Siberian gas venting: SGV model). In part this idea was supported by a U-Pb age of 252.0 ± 0.4 Ma for one of the dolerite sills in the southeastern Siberian Traps: the age corresponds to the Permo-Triassic boundary and its known mass extinctions of biota. In this study two other dolerite sills were dated using zircons by the U-Pb SHRIMP method at 254.2 ± 2.3 Ma and 249.6 ± 1.5 Ma. The former age is in agreement within error with the age previously published for the dolerite sills, whereas the latter age is in agreement with U-Pb ages published for lava and intrusions from the northern Siberian Traps. The new ages corresponds to the Cahngshingian/Wuchiapingian or Permian/Triassic and Spathian/Smithian boundaries, respectively. Review of 40Ar/39Ar and U-Pb SHRIMP ages previously published for the southeastern Siberian Traps shows that three other pulses of magmatism probably took place at respectively Anisian/Spathian, Late/Middle Anisian and Landian/Anisian boundaries. Thus it is possible that the SVG model can be applied also to lesser biotic extinctions and recoveries in proximity and aftermath to the main Permo-Triassic extinction.
"Vanadium-dravite" NaMg3V6(Si 6O18)(BO3)3(OH)3OH (IMA number 1999-050) has been redefined as oxy-vanadium-dravite with end-member formula NaV3(V4Mg2)Si6O18 (BO3)3(OH)3O. The new name and the new formula have been approved by the CNMNC (IMA proposal 11-E). Oxy-vanadium-dravite occurs in the metamorphic rocks of the Sludyanka complex (southern Baikal region, Russia). The crystal structure of oxy-vanadium-dravite has been refined for the first time using single-crystal X-ray data, with a statistical index R1 for all reflections converging to 1.44%. The structure is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1908(4), c = 7.4143(2) Å, V = 1683.21(7) Å3, Z = 3. The chemical characterization resulted in the empirical structural formula:(Equavation presented) Ideally, the oxy-vanadium-dravite is related to oxy-dravite and oxy-chromium-dravite by the homovalent substitution V3+ → Al and V3+ → Cr3+ (respectively) at the Y and Z sites. The occurrence of solid-solutions among V3+, Cr3+, and Al have been observed in tourmalines from metamorphic rocks of the Sludyanka complex. Significant chemical variations in V3+, Cr3+, and Al were also observed within zoned crystals from Sludyanka, not belonging to the holotype specimen
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