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Геология России

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X-RAY DIFFRACTION CRITERIA FOR THE IDENTIFICATION OF TRANS- AND CIS-VACANT VARIETIES OF DIOCTAHEDRAL MICAS
(2007) Zviagina B.B.; Sakharov B.A.; Drits V.A.
To provide structural and diffraction criteria for the identification of trans -vacant (tv) and cis -vacant ( cv ) mica varieties with different layer stackings, powder X-ray diffraction (XRD) patterns have been simulated for 1 M , 2 M 1 , 2 M 2 , 3 T and 2 O structural models consisting of either tv or cv layers. The differences in the unit-cell parameters resulting from the specific structural distortions of tv and cv layers lead to the differences in the positions of reflections having the same indices in the XRD patterns for tv and cv 1 M , 2 M 1 and 2 M 2 mica varieties. The tv 1 M , 2 M 1 and 2 M 2 varieties of Al-rich micas can therefore be distinguished from the corresponding cv varieties using powder XRD diffraction provided that the d values are measured with high precision and accurately compared with those calculated from the unit-cell parameters for the corresponding hkl indices. The differences in reflection positions for these tv and cv varieties should decrease with increasing Mg and/or Fe contents, thus complicating their identification. The peak positions and intensity distributions in the XRD pattern for the tv 3 T variety are similar to those for the cv 3 T structure with the vacancy in the right-hand cis site (3 T - cv 1), and both XRD patterns are similar to that for the 1 M - cv mica. The simulated XRD pattern for the cv 3 T structure with the vacancy in the left-hand cis site (3 T - cv 2) is similar to that for the 1 M - tv variety. The similarities and dissimilarities in intensity distribution between the XRD patterns simulated for the 1 M and 3 T varieties in question may be associated with the differences in the mutual arrangement of cations and anions in successive layers. Possible interstratification of tv and cv layers within the same structure should seriously complicate the identification of dioctahedral mica polytypes and polymorphs.
NOVEL 2:1 STRUCTURE OF PHYLLOSILICATES FORMED BY ANNEALING Fe3+, Mg-RICH DIOCTAHEDRAL MICA
(2007) Kogure T.; Kameda J.; Drits V.A.
A new modification of the 2 : 1 phyllosilicate layer has been discovered in annealed celaclonite, a Fe 31, Mg-rich dioctahedral mica. Plan-view diffraction patterns in TEM indicate a base-centered supercell with A = 3a and B = b, where a and b are the cell dimensions of the originat mica. Basic hOl reflections with h = 3n form an orthogonal lattice with one-layer periodicity, which is not expected for normal micas. The high-resolution TEM image along the (100) or related directions is similar to that expected from normal micas but the image along the (0 I 0) directions is completely different. From these images, it is concluded that the two tetrahedral sheets in a 2 : 1 layer are facing each other with no lateral a/3 stagger. In the proposed model that explains the high-resolution TEM images, two thirds of the spaces surrounded by two facing tetrahedral six-member rings accommodate three (Fe3+,Mg) cations and one third of the spaces are completely vacant. The (Fe3+,Mg) cations are coordinated by six or five oxygen atoms forming trigonal prisms or square pyramids, respectively.
HIGH-PRECISION PB ISOTOPE ANALYSIS BY MULTICOLLECTOR-ICP-MASS-SPECTROMETRY USING 205Tl/ 203Tl NORMALIZATION: OPTIMIZATION AND CALIBRATION OF THE METHOD FOR THE STUDIES OF Pb ISOTOPE VARIATIONS
(2007) Chernyshev I.V.; Chugaev A.V.; Shatagin K.N.
The development of the MC-ICP-MS method, which was launched about one decade ago and was largely stimulated by the need to solve geological problems, has opened a new avenue in isotope mass spectrometry. One of the advantages of this method is the possibility of applying a newly developed approach to the correction of analytical results for the effect of mass discrimination by normalizing the measured isotope ratios of an element to a reference (standard) isotope ratio of another element. This makes it possible to overcome the main disadvantage of conventional thermal ionization mass spectrometry (TIMS), in which the effect of mass discrimination cannot be fully taken into account during isotope analysis, and thus to implement a highly accurate method for the analysis of Pb-isotope composition. In application to the capability of the NEPTUNE MC-ICP mass spectrometer, we optimized and calibrated a method for high-accuracy Pb isotope analysis in solutions spiked with Tl, with all currently measured Pb-isotope ratios normalized to the standard 205Tl/203Tl ratio (TLN-MC-ICP-MS). The factors affecting the random and systematic analytical errors were examined, and the optimal operating regime and analytical conditions were determined. Much attention was paid to the correlation of the measurement results and the mass discrimination effect determined from the 205Tl/203Tl ratio. The value of the 205Tl/203Tl normalizing ratio was analytically determined through isotope analyses of the NIST SRM 981, and SRM 982 standard samples of Pb-isotope composition. The data obtained for two mixtures Tl + Pb (SRM 982) and Tl + Pb (SRM 981) in ten replicate analyses were 2.38898 ± 12 and 2.38883 ± 20, respectively. These results are in good mutual agreement, and their general mean 205Tl/203Tl = 2.3889 ± 1 coincides (within the error) with the recently published values of 2.3887 ± 7 [Collerson et al., 2002] and 2.3889 ± 1 [Thirlwall, 2002]. The precision of the method (±2SD), which was assayed by the long-term reproducibility of the results of replicate analyses of SRM 981 and seven galena samples (90 analyses) was 0.016–0.018% for the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios and 0.005 and 0.009% for the 207Pb/206Pb and 208Pb/206Pb ratios, respectively. The precision of the isotope analysis of common Pb was significantly improved (by factors of 6–10 for various isotope ratios) compared with the precision of TIMS techniques acceptable in isotope studies during three decades. The described method was applied to examine the Pb-isotope composition of approximately 250 samples of galena, scheelite, and pyrite from a number of well known (including large) gold, sulfied, and base-metal deposits. The precision of the method (0.01–0.02%) makes it possible to study small inter-and intra-phase differences in Pb-isotope ratios in hydrothermal and magmatic rocks, to assay the scale of regional and variations in the isotope composition of ore Pb, and to correlate the Pb-isotope composition of rocks and ores and reveal its evolutionary trends.
MANTLE(?) XENOLITHS IN THE CARBONATITES OF NORTHERN TRANSBAIKALIA
(2007) Ripp G.S.; Doroshkevich A.G.; Badmatsyrenov M.V.; Karmanov N.S.
The composition and nature of high-Cr minerals in lithic clasts from the carbonatites of the Veseloe occurrence, northern Transbaikalia, were considered. In order to determine their source, the Cr-bearing phases were compared with chromite, magnetite, and rutile from ultrabasic rocks, mantle xenoliths, and eclogites. It was suggested that the xenoclasts studied were formed at great depths, whereas the carbonatites were directly derived from the mantle rather than formed by the crustal differentiation of a silicate-carbonate melt.
ISOTOPIC THERMOMETRY OF SODALITE-BEARING MINERAL ASSOCIATIONS
(2007) Ustinov V.I.; Grinenko V.A.; Kotel'nikov A.R.
This publication is a logical continuation of our earlier isotopic studies of sodalite as a promising tool for the evaluation of temperatures and fluid regimes of mineral-forming processes [1]. The first data on the intrastructural oxygen isotopic distribution in synthetic sulfate-sodalite (nosean) Na8Al6Si6O24SO4 have demonstrated the possibility of using this mineral as a monomineralic isotopic geothermometer. Further exploring this avenue, we conducted a study of the infrastructural oxygen isotopic effects in Cl-sodalite and minerals coexisting with it in sodalitebearing mineral assemblages. The first task of our research was to determine the temperature dependence of the fractionation factor ∆18O in the Cl-sodalite–H2O system with the aim of evaluating the temperatures and nature of aqueous fluids.