L. Z. Reznitskii

Crystal chemistry of the dravite-chromdravite series

Five Cr-dravite-chromdravite samples, representative of the high-chromium part of the compositional field observed in tourmalines occurring in rocks surrounding Lake Baikal, were selected and studied by electron microprobe analysis and structural refinement. All examined tourmalines belong to the Alkali group, Oxy-subgroup. Their most striking feature is the exceptionally high Cr content (from 3.2 to 4.41 apfu), which substitutes for Al. The main divalent cation is Mg. Site populations were determined by a minimization procedure which simultaneously takes into account both structural and chemical data. Empirical bond distances for Y Cr 3+ -O (1.978 A) and z Cr 3+ -O (1.970 A) were optimised within the minimization procedure. Results indicate that the crystals are characterized by a quite disordered cation distribution between Y and Z octahedral sites, the various cations showing different degrees of preference. Cr 3+ and Mg populate both sites but show opposite behaviour: Mg has a marked preference for the Z octahedron and Cr 3+ for Y. Al almost exclusively populates Z. Most structural features are related to variations in the Z octahedron, whereas the dimensions of other polyhedra remain almost constant. Z dimensions depend on site populations, and are in particular determined by the z Cr 3+ ⟷ Z Al substitution. In the dravite-chromdravite series, both current and literature data show that the unit cell parameter c is strongly and positively correlated with Z dimensions, while correlation with a is evident only within the Cr-dravite-chromdravite subseries. Y does not actively participate in structural variations and, in particular, no correlation was ever observed between Y dimensions and unit cell parameters. Most structural variations are thus due to Z , while the effects of Y are negligible. This behaviour stands out when examining the a vs. c plot, for the whole series: a strong correlation between the two unit cell parameters was observed in the Cr-dravite-chromdravite subseries, but none within the dravite-Cr-dravite subseries or schorl-dravite series.

Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Abstract Vanadio-oxy-chromium-dravite, NaV3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with quartz, Cr-V-bearing tremolite and muscovite-celadonitechromphyllite- roscoelite, diopside-kosmochlor-natalyite, Cr-bearing goldmanite, escolaite-karelianite, dravite-oxy-vanadium-dravite, V-bearing titanite and rutile, ilmenite, oxyvanite-berdesinskiite, shreyerite, plagioclase, scapolite, zircon, pyrite, and an unnamed oxide of V, Cr, Ti, U, and Nb. Crystals are emerald green, transparent with a vitreous luster, pale green streak, and conchoidal fracture. Vanadio-oxy-chromium-dravite has a Mohs hardness of approximately 7½, and a calculated density of 3.3 g/cm3. In plane-polarized light, vanadio-oxy-chromium-dravite is pleochroic (O = dark green, E = pale green) and uniaxial negative: ω = 1.767(5), ε = 1.710(5). Vanadio-oxy-chromium-dravite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.1260(2), c = 7.3759(1) Å, V = 1661.11(4) Å3, Z = 3. Crystal chemistry analysis resulted in the empirical structural formula: X(Na0.89K0.06□0.05) Y(V3+2.77Mg0.17Fe3+0.06) Z(Cr3+ 1.85Al1.59V3+ 0.78Mg1.78) T[(Si5.95Al0.05)O18] B(BO3)3V(OH2.91O0.09) W(O0.86F0.14). The crystal structure of vanadio-oxy-chromium-dravite was refined to a statistical index R1 of 1.16% using 2543 unique reflections collected with MoKa X-radiation. Ideally, vanadio-oxy-chromiumdravite is related to oxy-chromium-dravite and oxy-vanadium-dravite by the homovalent substitution V3+ ↔ Cr3+. Tourmaline with chemical compositions classified as vanadio-oxy-chromium-dravite can be either Cr3+-dominant or V3+-dominant as a result of the compositional boundaries along the solid solution between Cr3+ and V3+ that are determined at Y+Z(V5Cr2), corresponding to NaY(V3)Z(V2Cr2Mg2) Si6O18(BO3)3(OH)3O, and Y+Z(V1.5Cr5.5), corresponding to NaY(V1.5Cr1.5)Z(Cr4Mg2)Si6O18(BO3)3(OH)3O.

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt, Southwestern Transbaikalia: Age and conditions of the formation

AbstractaThis paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.

Oxy-vanadium-dravite, NaV3(V4Mg2)(Si6O18)(BO3)3(OH)3O: Crystal structure and redefinition of the "vanadium-dravite" tourmaline

Abstract “Vanadium-dravite” NaMg3V6(Si6O18)(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: X(Na0.88K0.07□ 0.05)Y(V3+2.46Mg0.48Ti0.06)Z(V3+3.14Mg1.74Al0.91Cr3+0.21)T(Si5.99Al0.01O18)B(BO3)3V(OH)3W(O0.78OH0.14F0.08). 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.

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

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: X(Na0.76Ca0.19 □ 0.03K0.02)Y(Cr3+1.94Mg0.93Al0.07Ti4+0.06)Z(Al3.74Mg1.43Fe3+0.32V3+0.19Cr3+0.16Fe2+0.15) [T(Si5.99Al0.01)O18]B(BO3)3V(OH)3W[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)(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 chromoalumino- 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.

Formation history of the Tuva-Mongolian Massif (Western Hubsugul region, North Mongolia) based on U-Pb dating of detrital zircons from sandstone of the Darkhat group by the LA-ICP-MS method

1498 The Tuva–Mongolian Massif was defined in 1971 as a block with an Early Precambrian basement and Vendian–Cambrian carbonate cover of the platform type [1]. Subsequently, the contours of the massif were changed substantially and its dimensions were reduced [2]. The present day boundaries correspond to the contour outlining the distribution area of the Ven dian–Cambrian carbonate cover [3, 4] (Fig. 1). In the south, the Tuva–Mongolian Massif is separated by a fault from the Dzabkhan Massif. Formerly, these two massifs formed a single block, which is evident from the development of similar carbonate rocks in their covers [3]. In this connection, the validity of the Tuva–Mongolian Massif as a geological structure that developed against the background of surrounding younger Paleozoic complexes is beyond doubt [5]. The age of blocks constituting the basement of the massif, as well as the history of their amalgamation and rela tion with other blocks, remains insufficiently known despite the availability of many special publications dedicated to these aspects [2–4, 6–9]. In this commu nication, we present the data on ages of zircons from sandstone of the Darkhat Group underlying the Ven dian–Cambrian carbonate rocks on the eastern mar gin of the central Tuva–Mongolian Massif. It appeared that these detrital zircons adequately char acterize the provenance and reflect practically the entire pre Vendian formation history of the massif.

Crystal chemistry of Cr3+-V3+-rich clinopyroxenes

Abstract Eleven clinopyroxenes from the Sludyanka Crystalline Complex in Russia belonging to the ternary join NaVSi2O6-NaCrSi2O6-CaMgSi2O6 (natalyite-kosmochlor-diopside) were studied by means of X-ray single crystal diffractometry and electron probe microanalysis. The crystal chemical data show that the T site is almost completely occupied by Si, so that the Na (V3+,Cr3+) → Ca Mg substitution mechanism ensures charge balance. Changes in M1 site geometry are explained by the aggregate ionic radius, and are influenced by Mg occupancy and V3+/(V3+ + Cr3+) ratio. The M2 site geometry depends both on Na content and on the (V3+,Cr3+) → Mg substitution in M1 site. Changes in M2-O3c1 bond length are mainly related to Na content, whereas the longest M2-O3c2 bond lengths are significantly affected by the V3+/(V3+ + Cr3+) ratio of the M1 site. The T site geometry is affected by chemical and geometrical variations at the M1 and M2 sites, principally the M1 site occupancy.

Chromo-alumino-povondraite, NaCr3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

Abstract Chromo-alumino-povondraite, NaCr3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral of the tourmaline supergroup. It is found in metaquartzites of the Pereval marble quarry (Sludyanka, Lake Baikal, Russia) in association with dravite, oxy-chromium-dravite, oxy-dravite, quartz, calcite, chromphyllite, eskolaite, chromite, uvarovite, chromian phlogopite, and pyroxenes of the diopside-kosmochlor series, Cr-bearing tremolite, Cr-bearing titanite, Cr-bearing rutile, and pyrite. Crystals are green and transparent with a vitreous luster, and exhibit a pale-green streak and conchoidal fracture. Chromo-alumino-povondraite has a Mohs hardness of approximately 7½, and a calculated density of 3.227 g/cm3. In plane-polarized light, chromo-alumino-povondraite is pleochroic (O = emerald green and E = pale yellowish green) and uniaxial negative: ω = 1.745(5), ε = 1.685(5). Chromo-alumino-povondraite is rhombohedral, space group R3m, with the unit-cell parameters a = 16.0277(2), c = 7.3085(1) Å, V = 1625.93(5) Å3, Z = 3. Crystal-chemical analysis resulted in the empirical structural formula: X(Na0.87Ca0.07□0.04K0.02)Σ1.00Y(Cr3+2.29Mg0.71)Σ3.00Z(Al3.04Mg1.54Cr3+1.18V3+0.22Fe3+0.01)Σ6.00 [T(Si5.96Al0.04)O18] (BBO3)3V(OH)3W[O0.73F0.25(OH)0.02]Σ1.00 The crystal structure of chromo-alumino-povondraite was refined to an R1 index of 1.68% using 1803 unique reflections collected with MoKα X-radiation. Ideally, chromo-alumino-povondraite is related to oxy-dravite and oxy-chromium-dravite by the homovalent substitution Cr3+ ↔ Al3+. Tourmaline with chemical compositions classified as chromo-alumino-povondraite can be either Al-dominant or Cr-dominant as a result of the compositional boundaries along the solid solution between Al and Cr3+ that are determined at Y+Z(Cr1.5Al5.5), corresponding to NaY(Cr1.5Al1.5)Z(Al4Mg2)Si6O18(BO3)3(OH)3O, and Y+Z(Cr5Al2), corresponding to NaY(Cr3)Z(Cr2Al2Mg2)Si6O18(BO3)3(OH)3O.

Catastrophic outburst and tsunami flooding of Lake Baikal: U–Pb detrital zircon provenance study of the Palaeo-Manzurka megaflood sediments

ABSTRACT Lake Baikal, the largest freshwater reservoir on Earth (~600 × 30 km in size and up to 1.6 km in depth), has more than 300 contributing rivers but only one N-trending outflow – River Angara. In the Pliocene or Pleistocene, another N-trending outflow operated through the Palaeo-Manzurka to Lena. Provenance analysis using U–Pb dating of detrital zircons from the Palaeo-Manzurka sediments demonstrates that the dominant source of the zircons was the lake deposits, while the contribution of zircons from local bedrocks was limited to about 8% only. Looking for an explanation of this, we propose a hypothesis that formation of the Palaeo-Manzurka sediments took place in association with a catastrophic mega-landslide (~15 × 3 km) into the lake and the resulting mega-tsunami flooding.

New data about structure and time of formation of the Khamar-Daban terrane: U-Pb LA-ICP-MS zircon ages

This paper provides new data on the age of detrital zircons from metaterrigenous rocks of the Khamardaban Group (Kornilovskaya and Shubutuiskaya formations) which crown the succession of the Khamar-Daban terrain. It was established that the accumulation of the protoliths of both formations occurred in the interval between the Late Riphean and Early Ordovician. In this case, there is a difference between sequences of Shubutuiskaya and Kornilovskaya formations due to a sharp change in a provenance area and depositional conditions, which is a consequence of the change in the paleogeodynamic environment. In addition, this indicates the tectonic juxtaposition of these sequences and probability of accumulation of deposits of the Shubutuiskaya Formation within Dzhida island arc system.