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Naotaka Tomioka Takuo Okuchi Narangoo Purevjav Jun Abe Stefanus Harjo 《Physics and Chemistry of Minerals》2016,43(4):267-275
Hydrogen site positions and occupancy in the crystal structure of dense hydrous magnesium silicate (DHMS) phase E were determined for the first time by pulsed neutron powder diffraction. A fully deuterated pure phase E powder sample, which had space group \(R\overline{3} m\) and lattice parameters of a = 2.97065(8) Å and c = 13.9033(4) Å, was synthesized at 15 GPa and 1100 °C. Through quantitative evaluation of refined structure parameters obtained with sufficient spatial resolution and very high signal-to-background ratio, we conclude that the O–D dipoles in the refined phase E structure are tilted by 24° from the direction normal to the layers of edge-shared MgO6 octahedra (octahedral layers). The tilted dipole structure of phase E is in remarkable contrast to that of brucite, Mg(OH)2, which has dipoles exactly normal to the octahedral layer. This contrast exists because the O–Si–O bonding unique in the phase E structure connects two adjacent octahedral layers and thereby reduces the interlayer O···O distance. This shrinkage of the interlayer distance induces the tilting of the O–D dipole and also generates unique O–D···O hydrogen bonding connecting all the layers in the phase E structure. 相似文献
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The Devonian–Carboniferous Tsetserleg terrane of Mongolia forms part of the complex Central Asian Orogenic Belt (CAOB). The Tsetserleg terrane consists mainly of clastic sediments, and is situated in the southern Hangay–Hentey Basin. Internally the terrane is divided into the Erdenetsogt (Middle Devonian), Tsetserleg (Middle‐Upper Devonian) and Jargalant (Lower Carboniferous) Formations. Provenance and tectonic setting of the Hangay–Hentey Basin remains controversial, with proposals ranging from passive margin through to island‐arc. A suite of 94 Tsetserleg sandstones and mudrocks was collected with the aim of constraining provenance, source weathering, and depositional setting, using established petrographic and whole‐rock geochemical parameters. Petrographically the sandstones are immature, with average compositions of Q22F14L64, Q14F17L69, and Q18F12L70 in the Erdenetsogt, Tsetserleg, and Jargalant Formations, respectively. Lv/L ratios range from 0.81 to 1.00 (average 0.95), and P/F from 0.68 to 0.93 (average 0.83). Framework compositions indicate deposition in an undissected or transitional arc. Geochemically, the sandstones are classified as greywackes. Geochemical contrasts between sandstone and mudrock averages in each formation are small, with lithotype means for SiO2 ranging only from 65.54 to 68.62 wt.%. These features and weak trends on variation diagrams reflect the immaturity of the sediments. Comparison of elemental abundances with average upper continental crust, major element discriminant scores, and immobile element ratios indicate a uniform average source composition between dacite and rhyolite. The maximum value for the Chemical Index of Alteration in the Erdenetsogt Formation is about 78 after correction for K‐metasomatism, indicating moderate source weathering. Lower maximum values (61 and 63, respectively) in the Tsetserleg and Jargalant Formations indicate they were derived from a virtually unweathered and tectonically active source. Tectonic setting discrimination parameters indicate a continental island‐arc environment, similar to several other CAOB suites of similar age. This arc source may have been built on a continental fragment situated within the Mongol–Okhotsk Ocean during Middle Devonian‐Lower Carboniferous time. 相似文献
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The Hangay–Hentey belt is situated in the central Northern Mongolia, and forms part of the Central Asian Orogenic Belt (CAOB). It is internally subdivided into seven terranes, the largest of which are the neighbouring Ulaanbaatar and Tsetserleg terranes. These coeval terranes are mainly composed of Silurian–Devonian accretionary complexes and Carboniferous turbidites. Proposals for their depositional setting range from passive margin through to island arc. A suite of 19 Ulaanbaatar terrane sandstones and mudrocks (Gorkhi and Altanovoo Formations) were collected with the aim of constraining their provenance, source weathering, and depositional setting based on whole-rock major and trace element data, and for comparison with the neighbouring Tsetserleg terrane. New REE analyses were also made of 35 samples from the Ulaanbaatar and Tsetserleg terranes. Geochemically the Ulaanbaatar sandstones are classed as wackes, and most of the mudstones as shales. Geochemical parameters suggest an immature source, similar to that of the Tsetserleg terrane. Geochemical contrasts between sandstones and mudrocks in the Ulaanbaatar sediments are small, and trends on element – Al2O3 variation diagrams are weak. Comparison with average upper continental crust (UCC), major element discriminant scores, and immobile element ratios (Th/Sc, Zr/Sc, Ce/Sc, Ti/Zr) indicate a uniform average source composition between dacite and rhyolite. Maximum Chemical Index of Alteration value in the Ulaanbaatar terrane is ∼65 after correction for K-metasomatism, indicating minimal weathering in a tectonically active source, similar to that of the Tsetserleg terrane. REE data in both terranes show moderate LREE enrichment and flat HREE segments, with negative Eu anomalies somewhat less than those in UCC and PAAS. Chondrite-normalized patterns are very similar to that for average Paleozoic felsic volcanic rock, supporting the relatively felsic source indicated by immobile trace element ratios. Tectonic setting discriminants (K2O/Na2O–SiO2/Al2O3, La–Th–Sc, Th–Sc–Zr) indicate an evolved continental island arc (CIA; A2) environment for both terranes, similar to several other CAOB suites of similar age. This common arc source was situated within the Mongol-Okhotsk Ocean during Silurian–Lower Carboniferous time. The present-day Aleutian arc is a possible modern analogue of the depositional setting. 相似文献
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