Local structure and chemical shifts for six-coordinated silicon in high-pressure mantle phases

Most of the earth's mantle is made up of high-pressure silicate minerals that contain octahedrally coordinated silicon (Si(VI)), but many thermodynamically important details of cation site ordering remain unknown. Silicon-29 nuclear magnetic resonance (NMR) spectroscopy is potentially very useful for determining short-range structure. A systematic study of silicon-29 chemical shifts for Si(VI) has revealed empirical correlations between shift and structure that are useful in understanding several new calcium silicates. The observed ordering state of a number of high-pressure magnesium silicates is consistent with the results of previous x-ray diffraction studies.     

The nature of the glass transition in a silica-rich oxide melt

The atomic-scale dynamics of the glass-to-liquid transition are, in general, poorly understood in inorganic materials. Here, two-dimensional magic angle spinning nuclear magnetic resonance spectra collected just above the glass transition of K(2)Si(4)O(9) at temperatures as high as 583 degrees C are presented. Rates of exchange for silicon among silicate species, which involves Si-O bond breaking, have been measured and are shown to be closely related in time scale to those defined by viscosity. Thus, even at viscosities as high as 10(10) pascal seconds, local bond breaking (in contrast to the cooperative motion of large clusters) is of major importance in the control of macroscopic flow and diffusion.     

A stable silicon(0) compound with a Si=Si double bond

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 +/- 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si-Si(Cl):L and L:Si=Si:L.     

A linear, O-coordinated eta1-CO2 bound to uranium

The electron-rich, six-coordinate tris-aryloxide uranium(III) complex [((AdArO)3tacn)U(III)] [where (AdArOH)3tacn = 1,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane] reacts rapidly with CO2 to yield [((AdArO)3tacn)U(IV)(CO2)], a complex in which the CO(2) ligand is linearly coordinated to the metal through its oxygen atom (eta1-OCO). The latter complex has been crystallographically and spectroscopically characterized. The inequivalent O-C-O bond lengths [1.122 angstroms (A) for the O-C bond adjacent to uranium and 1.277 A for the other], considered together with magnetization data and electronic and vibrational spectra, support the following bonding model: U(IV)=O=C*-O- <--> U(IV)-OC-O-. In these charge-separated resonance structures, the uranium center is oxidized to uranium(IV) and the CO2 ligand reduced by one electron.     

Upper atmosphere and ionosphere of Mars

It is argued that the single-layer ionosphere at 125 kilometers discovered in the Mariner IV occultation experiment is an Fl region coinciding with the ultraviolet photoionization peak. The CO(2) density there must be of the order of 10(11) molecules per cubic centimeter. Such a density is consistent with the properties of the lower atmosphere by Mariner IV anid the temperature model of Chamberlain and McElroy if the atmosphere is mainly CO(2) below 70 kilometers. The absence of an F2 region can be explained even if the density ratio of O to CO(2) is 100 at 230 kilometers on the basis of the rapid conversion of O(+) to O(2) by CO(2). Thus a model with an exospheric temperature of 400 degrees K, a modest degree of CO(2) dissociation, and diffusive separation above 70 kilometers is possible.     

Phase velocities of rayleigh waves in the MELT experiment on the east pacific rise

The phase velocities of Rayleigh waves increase more rapidly with distance from the East Pacific Rise (EPR) axis than is predicted by models of conductive cooling of the lithosphere. Low velocities near the axis are probably caused by partial melt at depths of 20 to 70 kilometers in a zone several hundred kilometers wide. The lowest velocities are offset to the west of the EPR. Wave propagation is anisotropic; the fast direction is approximately perpendicular to the ridge, parallel to the spreading direction. Anisotropy increases from a minimum near the axis to 3 percent or more on the flanks.     

Spectroscopic evidence for pressure-induced coordination changes in silicate glasses and melts

Infrared spectra demonstrate that at pressures above 20 gigapascals and room temperature the regular tetrahedral coordination of oxygen around both silicon and aluminum ions is severely disrupted in SiO(2), CaMgSi(2)O(6), and CaAlSi(2)O(8) composition glasses. The spectra are consistent with gradual, pressure-induced increases in the coordination numbers of silicon and aluminum. A variety of coordination environments, from sixfold to fourfold, appears to be present at pressures as high as about 40 gigapascals. This apparent change in coordination is not quenchable at room temperature: on decompression, the glasses return to tetrahedral coordination. This continuous and reversible coordination change in amorphous silicates explains the lack of observation of coordination changes in silicate glasses quenched from high pressure, the shallow melting slopes observed for mantle silicates at high pressures, and the possible presence of neutrally buoyant magmas deep within the terrestrial planets.     

Spin transition zone in Earth's lower mantle

Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.     

An octahedral coordination complex of iron(VI)

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO4(2-). We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. M?ssbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet d2(xy) ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.     

Effect of a Coordination Change on the Strength of Amorphous SiO2

Measurements of the yield strength of SiO(2) glass to pressures as high as 81 gigapascals at room temperature show that the strength of amorphous silica decreases significantly as it is compressed to denser strctures with higher coordination. Above 27 gigapascals, as the silicon in amorphous SiO(2) is continuously transformed from fourfold to sixfold coordination, the strength of the glass decrases by more than an order of magnitude. These data confirm theoretical predictions that the mechanical properties of polymerized amorphous silicates are sensitive to pressure-induced structural transformations and suggest that the viscosity of silica-rich liquids decreases significantly at high pressures. Such a change in melt rheology could enhance the processes of chemical differentiation with depth in the Earth's mantle.     

Ultraviolet observations of solar fine structure

The High Resolution Telescope and Spectrograph was flown on the Spacelab-2 shuttle mission to perform extended observations of the solar chromosphere and transition zone at high spatial and temporal resolution. Ultraviolet spectroheliograms show the temporal development of macrospicules at the solar limb. The C IV transition zone emission is produced in discrete emission elements that must be composed of exceedingly fine (less than 70 kilometers) subresolution structures.     

Iron catalysts for selective anti-Markovnikov alkene hydrosilylation using tertiary silanes

Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino)pyridine ligands, show promise for industrial application.     

基于耦合协调模型的安徽省城镇化发展与土地生态安全关系评价研究

城镇化是我国社会经济发展的必然产物,土地生态安全是生态文明建设的重要内容之一,城镇化进程的加快使得土地生态安全问题受到越来越多的关注。文章基于熵权法和耦合协调度模型分析安徽省2000—2018年城镇化发展与土地生态安全综合评价指数和耦合协调度,利用灰色预测模型进行预测。结果表明:(1)安徽省城镇化和土地生态安全发展水平在2000—2018年呈明显的上升趋势,现阶段属于土地生态安全滞后型;(2)城镇化和土地生态安全综合评价指数稳步增加,耦合度为拮抗状态,两系统耦合协调类型从极度失调衰退逐渐演变为初级协调发展;(3)预测2019—2023年安徽省城镇化和土地生态安全耦合协调度呈稳步上升的发展趋势,在2023年可能达到良好协调发展的水平。为推动安徽省城镇化与土地生态安全协调发展水平,建议优化产业结构,转变经济发展方式、加大环保投入、树立一体化发展理念等。     

京津冀地区生态基础设施?生境质量?产业发展耦合协调性分析与预测

  目的  对生态环境与产业发展耦合协调关系进行研究,有助于规划者从生态学角度宏观把握京津冀协同发展趋势,为规划与管理提供科学依据。  方法  该研究基于MSPA和InVEST模型分析了京津冀地区2000—2017年生态基础设施、生境质量和产业发展变化特征,并采用耦合协调模型对该地区三者的耦合协调性进行综合分析,最后利用灰色预测模型预测京津冀不同区域未来五年耦合协调度。  结果  （1）京津冀地区生态基础设施水平整体偏低,连通性较差;生境质量空间差异显著,整体生境质量0.450,水平不高;（2）西北部生态涵养区的农林牧渔业发展稳定,津冀工业长期处于主导地位,东部滨海发展区交通运输、仓储和邮政业和京津金融业发展实力较强;（3）京津冀地区耦合协调度主要处于轻度失调到初级协调状态之间,空间差异明显。其中,南部功能拓展区耦合协调度最高,而西北部生态涵养区耦合协调度最低。（4）未来5年京津冀地区耦合协调度均将有所提升,但中部核心功能区仍将处于濒临失调状态,北京和东部滨海发展区仍将处于勉强协调状态,是未来需要优化的重点区域;天津耦合协调度将由勉强协调转变为初级协调,西北部生态涵养区耦合协调度将由濒临失调转变为初级协调,是具有协调发展潜力的区域;南部功能拓展区耦合协调类型将转变为中级协调,是发展速度最快的区域。  结论  该研究通过评估京津冀区域生态基础设施?生境质量?产业发展的耦合协调性,并分析预测具有协同发展潜力的重点区域,为未来京津冀协同发展提供依据,并为其他城市群范围内生态建设和产业发展评估提供重要参考。      

Tuhualite crystal structure

As judged by crystal structure analysis, the crystallochemical formula of tuhualite is (Na,K)(2)(VI) (Fe(+3))(2)(VI) (Fe(+2))(2)(VI) Si(12)O(30) . H(2)O, with four such formulas in the unit cell. The structure is characterized by six-repeat double chains of silicon tetrahedrons and chains of edge-sharing, alternating iron tetrahedrons and octahedrons. Chains of each kind are connected by corner sharing to four chains of the other type. The iron cations appear to be anomalously ordered with Fe(+2) in tetrahedral and Fe(+3) in octahedral sites.     

Three-dimensional structure at the base of the mantle beneath the central pacific

Forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central Pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models. A broad zone of low S velocity (-4 percent with respect to standard models), which may correspond to the base of a thermal "plume," narrows and is deflected as it extends to about 1000 kilometers above the core-mantle boundary. To the east of this zone, a localized region of fast S velocity (+5 percent) suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages, which may or may not involve core material. Its presence could also explain the observation of precursors to core reflected phases in this region.     

Ultradeep (>300 kilometers) ultramafic xenoliths: petrological evidence from the transition zone

The seismologically delineated transition zone, at depths between 400 and 670 kilometers, is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle. Xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite. These xenoliths show evidence for the high-pressure (90 to 120 kilobars) transformation of pyroxene to a solid solution of pyroxene in garnet (majorite) and silicon in octahedral coordination; low-pressure (less than 80 kilobars) exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved. Although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths, it is likely that both majorite and beta-spinel help produce the observed seismic gradient of the transition zone.     

Covalent Group IV Atomic Clusters

Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C(n)(+) clusters, which tend to fragment with a characteristic loss of a neutral C(3), are found to be different from the modes for Si(n)(+) and Ge(n)(+) clusters, which tend to fragment to "magic" clusters such as Si(4)(+), Si(6)(+) and Si(10)(+). These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences.     

施硅降低碱性土壤铅生物有效性的机制研究

为明确硅降低碱性土壤铅生物有效性的机制,在中和硅酸钠碱性和消除钠离子影响的基础上,采取吸附-解吸试验、红外分析和盆栽试验相结合的方式,研究了硅对碱性土壤液相、固相和固液界面铅行为以及水稻产量与稻米铅含量的影响。结果表明,加硅降低了碱性土壤吸附铅的容量和强度,增强了铅吸附-解吸过程中的滞后效应;应用红外光谱表征溶液中硅铅反应产物,发现Si-O弯曲振动吸收峰和Si-O-Si反对称伸缩振动特征峰均出现了红移,暗示硅酸或聚硅酸与铅发生了配位反应,生成了水溶性复合物;施硅使土壤酸可提取态和可还原态铅含量分别降低了11.18%、18.54%;可氧化态和残渣态铅分别提高了42.56%、7.84%;与对照相比,铅胁迫下水稻产量降低了64.5%(P0.01),糙米铅含量明显升高(P0.01),达到了0.31 mg·kg-1;与铅胁迫处理相比,施硅使水稻产量提高了152.3%(P0.01),明显降低糙米铅的含量(P0.01),并符合国家标准的安全要求。综上,在碱性土壤中,促进土壤液相、固相铅向无效态转化,抑制固相铅解吸是施硅降低铅生物有效性的土壤化学机制。     

Voyager 2 radio science observations of the uranian system: atmosphere, rings, and satellites

Voyager 2 radio occultation measurements of the Uranian atmosphere were obtained between 2 and 7 degrees south latitude. Initial atmospheric temperature profiles extend from pressures of 10 to 900 millibars over a height range of about 100 kilometers. Comparison of radio and infrared results yields mole fractions near the tropopause of 0.85 and 0.15 +/- 0.05 for molecular hydrogen and helium, respectively, if no other components are present; for this composition the tropopause is at about 52 kelvins and 110 millibars. Distinctive features in the signal intensity measurements for pressures above 900 millibars strongly favor model atmospheres that include a cloud deck of methane ice. Modeling of the intensity measurements for the cloud region and below indicates that the cloud base is near 1,300 millibars and 81 kelvins and yields an initial methane mole fraction of about 0.02 for the deep atmosphere. Scintillations in signal intensity indicate small-scale stucture throughout the stratosphere and upper troposphere. As judged from data obtained during occultation ingress, the ionosphere consists of a multilayer structure that includes two distinct layers at 2,000 and 3,500 kilometers above the 100-millibar level and an extended topside that may reach altitudes of 10,000 kilometers or more. Occultation measurements of the nine previously known rings at wavelengths of 3.6 and 13 centimeters show characteristic values of optical depth between about 0.8 and 8; the maxim value occurs in the outer region of the in ring, near its periapsis. Forward-scattered signals from this ring have properties that differ from those of any of Saturn's rings, and they are inconsistent with a discrete scattering object or local (three-dimensional) assemblies of orbiting objects. These signals suggest a new kdnd of planetary ring feature characterized by highly ordered cylindrical substructures of radial scale on the order of meters and azimuthal scale of kilometers or more. From radio data alone the mass of the Uranian system is GM(sys) = 5,794,547- 60 cubic kilometers per square second; from a combination of radio and optical navigation data the mass of Uranus alone is GM(u) = 5,793,939+/- 60 cubic kilometers per square second. From all available Voyager data, induding imaging radii, the mean uncompressed density of the five major satellites is 1.40+/- 0.07 grams per cubic centimeter; this value is consistent with a solar mix of material and apparently rules out a cometary origin of the satellites.