Structure and Density of FeS at High Pressure and High Temperature and the Internal Structure of Mars

In situ x-ray diffraction measurements revealed that FeS, a possible core material for the terrestrial planets, transforms to a hexagonal NiAs superstructure with axial ratio (c/a) close to the ideal close-packing value of 1.63 at high pressure and high temperature. The high-pressure-temperature phase has shorter Fe-Fe distances than the low-pressure phase. Significant shortening of the Fe-Fe distance would lead to metallization of FeS, resulting in fundamental changes in physical properties of FeS at high pressure and temperature. Calculations using the density of the high-pressure-temperature FeS phase indicate that the martian core-mantle boundary occurs within the silicate perovskite stability field.     

An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater

We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.     

Natural NaAlSi(3)O(8)-hollandite in the shocked sixiangkou meteorite

The hollandite high-pressure polymorph of plagioclase has been identified in shock-induced melt veins of the Sixiangkou L6 chondrite. It is intimately intergrown with feldspathic glass within grains previously thought to be "maskelynite." The crystallographic nature of the mineral was established by laser micro-Raman spectroscopy and x-ray diffraction. The mineral is tetragonal with the unit cell parameters a = 9.263 +/- 0.003 angstroms and c = 2.706 +/- 0.003 angstroms. Its occurrence with the liquidus pair majorite-pyrope solid solution plus magnesiowustite sets constraints on the peak pressures that prevailed in the shock-induced melt veins. The absence of a calcium ferrite-structured phase sets an upper bound for the crystallization of the hollandite polymorph near 23 gigapascals.     

The widespread distribution of a novel silica polymorph in microcrystalline quartz varieties

An x-ray examination of more than 150 specimens of fine-grained quartz varieties from around the world has revealed that more than 10% and as much as 80% of the silica in many samples is actually moganite, a little-known silica polymorph. Rietveld refinements of 50 powder x-ray diffraction patterns produced by fibrous quartz (agate, chalcedony) and nonfibrous quartz (chert, flint) indicate that the concentrations of moganite within each subgroup are widely distributed. The large amount of moganite (>30%) found in cherts from arid, alkaline environments may resurrect length-slow silica as an indicator of evaporitic regimes, and the absence of moganite in weathered and hydrothermally altered silica samples may be a useful measure of fluid-rock interaction.     

Modified spinel, Beta-manganous orthogermanate: stability and crystal structure

A new high-pressure polymorph with a modified spinel structure, beta-Mn(2)GeO(4), is stable in a pressure range intermediate between the field of the polymorph with the olivine structure and that of another high-pressure polymorph. Oxygen atoms are located approximately in cubic close packing with manganese and germanium atoms in octahedral and tetrahedral interstices, respectively, as in the spinel structure; however, germanium atoms form Ge(2)O(7) groups instead of isolated GeO(4) groups.     

A monoclinic post-stishovite polymorph of silica in the shergotty meteorite

A post-stishovite phase of silica was identified in the Shergotty meteorite by x-ray diffraction and field emission scanning electron microscopy. The diffraction pattern revealed a monoclinic lattice, similar to the baddeleyite-structured polymorph with the cell parameters a = 4.375(1) angstroms, b = 4.584(1) angstroms, c = 4. 708(1) angstroms, beta= 99.97(3), rho = 4.30(2) grams per cubic centimeter, where the numbers in parentheses are the maximum deviations. Transmission electron microscopy investigations indicate the presence of the alpha-lead dioxide-like polymorph, stishovite, and secondary cristobalite in the same silica grain. The mixture of high-density polymorphs suggests that several post-stishovite phases were formed during the shock event on the Shergotty parent body.     

Stability of Perovskite (MgSiO3) in the Earth's Mantle

Available thermodynamic data and seismic models favor perovskite (MgSiO3) as the stable phase in the mantle. MgSiO3 was heated at temperatures from 1900 to 3200 kelvin with a Nd-YAG laser in diamond-anvil cells to study the phase relations at pressures from 45 to 100 gigapascals. The quenched products were studied with synchrotron x-ray radiation. The results show that MgSiO3 broke down to a mixture of MgO (periclase) and SiO2 (stishovite or an unquenchable polymorph) at pressures from 58 to 85 gigapascals. These results imply that perovskite may not be stable in the lower mantle and that it might be necessary to reconsider the compositional and density models of the mantle.     

Enhancement of cation diffusion rates across the 410-kilometer discontinuity in Earth's mantle

Rates of cation diffusion (magnesium, iron, and nickel) have been determined in olivine and its high-pressure polymorph, wadsleyite, at 9 to 15 gigapascals and 1100 degrees to 1400 degreesC for compositions that are relevant to Earth's mantle. Diffusion in olivine becomes strongly dependent on composition at high pressure. In wadsleyite, diffusion is one to two orders of magnitude faster than in olivine, depending on temperature. Homogenization of mantle heterogeneities (chemical mixing) and mineral transformations involving a magnesium-iron exchange will therefore occur considerably faster in the transition zone than at depths of less than 410 kilometers.     

塔河油田集输管道内FeS的形成及自燃风险

塔河油田集输管道输送介质具有高含H,s、高含CO,及高含氯离子的特点,在无氧且低流速环境下,易生成FeS等腐蚀产物,FeS具有自燃性,一旦发生自燃将会引发火灾和爆炸事故。采用X-射线衍射测试方法对腐蚀产物组分进行分析,在H2S、CO2共存体系下,腐蚀产物以FeCO3、FeO（OH）、Fe3O4为主,铁的硫化物以FeS为主,但含量较低。采用加热的方式,利用固体自燃点测试仪对不同组分的7个腐蚀产物混合物进行自燃特性分析,结果表明：FeCO。可以表现出一定放热,但不会发生自燃;对于FeCO3与FeS形成的混合物,当两者质量比为15：10、单质硫磺的质量分数为2．67％时,FeCO。和FeS混合物自燃点为189．08℃,易发生自燃,建议在现场断管作业中采取预防FeS自燃的措施。（图2,表2,参6）。     

High-pressure chemistry of hydrogen in metals: in situ study of iron hydride

Optical observations and x-ray diffraction measurements of the reaction between iron and hydrogen at high pressure to form iron hydride are described. The reaction is associated with a sudden pressure-induced expansion at 3.5 gigapascals of iron samples immersed in fluid hydrogen. Synchrotron x-ray diffraction measurements carried out to 62 gigapascals demonstrate that iron hydride has a double hexagonal close-packed structure, a cell volume up to 17% larger than pure iron, and a stoichiometry close to FeH. These results greatly extend the pressure range over which the technologically important iron-hydrogen phase diagram has been characterized and have implications for problems ranging from hydrogen degradation and embrittlement of ferrous metals to the presence of hydrogen in Earth's metallic core.     

Synchrotron X-ray Diffraction Measurements of Single-Crystal Hydrogen to 26.5 Gigapascals

The crystal structure and equation of state of solid hydrogen have been determined directly to 26.5 gigapascals at room temperature by new synchrotron x-ray diffraction techniques. Solid hydrogen remains in the hexagonal close-packed structure under these pressure-temperature conditions and exhibits increasing structural anisotropy with pressure. The pressure-volume curve determined from the x-ray data represents the most accurate experimental measurement of the equation of state to date in this pressure range. The results remove the discrepancy between earlier indirect determinations and provide a new experimental constraint on the molecular-to-atomic transition predicted at higher pressures.     

Lead: X-ray Diffraction Study of a High-Pressure Polymorph

An x-ray diffraction study of lead under pressure has shown that face-centered cubic structure transforms to the hexagonal close-packed structure at room temperature and a pressure of 130+/- 10 kilobars. The volume change for the transformation is -0.18+/- 0.06 cubic centimeter per mole.     

Synthesis and Structure of an Iron(III) Sulfide-Ferritin Bioinorganic Nanocomposite

Amorphous iron sulfide minerals containing either 500 or 3000 iron atoms in each cluster have been synthesized in situ within the nanodimensional cavity of horse spleen ferritin. Iron-57 M?ssbauer spectroscopy indicated that most of the iron atoms in the 3000-iron atom cores are trivalent, whereas in the 500-iron atom clusters, approximately 50 percent of the iron atoms are Fe(III), with the remaining atoms having an effective oxidation state of about +2.5. Iron K-edge extended x-ray absorption fine structure data for the 500-iron atom nanocomposite are consistent with a disordered array of edge-shared FeS(4) tetrahedra, connected by Fe(S)(2)Fe bridges with bond lengths similar to those of the cubane-type motif of iron-sulfur clusters. The approach used here for the controlled synthesis of bioinorganic nanocomposites could be useful for the nanoscale engineering of dispersed materials with biocompatible and bioactive properties.     

Strength of diamond

The yield strength of diamond is measured under a pressure of 10 gigapascals at temperatures up to 1550 degrees C by the analysis of x-ray peak shapes on diamond diffraction lines in a powdered sample as a function of pressure and temperature. At room temperature, the diamond crystals exhibit elastic behavior with increasing pressure. Significant ductile deformation is observed only at temperatures above 1000 degrees C at this pressure. The differential yield strength of diamond decreases with temperature from 16 gigapascals at 1100 degrees C to 4 gigapascals at 1550 degrees C. Transmission electron microscopy observations on the recovered sample indicate that the dominant deformation mechanism under high pressure and temperature is crystal plasticity.     

Polymorphs of Alumina Predicted by First Principles: Putting Pressure on the Ruby Pressure Scale

Fully optimized quantum mechanical calculations indicate that Al2O3 transforms from the corundum structure to the as yet unobserved Rh2O3 (II) structure at about 78 gigapascals, and it further transforms to Pbnm-perovskite structure at 223 gigapascals. The predicted x-ray spectrum of the Rh2O3 (II) structure is similar to that of the corundum structure, suggesting that the Rh2O3 (II) structure could go undetected in high-pressure x-ray measurements. It is therefore possible that the ruby (Cr3+-doped corundum) fluorescence pressure scale is sensitive to the thermal history of the ruby chips in a given experiment.     

Crystal Structures Adopted by Black Phosphorus at High Pressures

Black phosphorus undergoes two reversible structural transitions at high pressures. The first is to a structure of the type arsenic A7. This structure is transformed to a simple cubic structure at higher pressures. The reversibility between the A7 and simple cubic structures at 111 kilobars indicates that the transition obtainable at this pressure provides a good calibration point by which high-pressure x-ray data may be united with volumetric or resistance measurements, or both.     

X-ray Variability in the Hot Supergiant zgr Orionis

Hot massive stars represent only a small fraction of the stellar population of the galaxy, but their enormous luminosities make them visible over large distances. Therefore, they are ideal standard candles, used to determine distances of near galaxies. Their mass loss due to supersonic winds driven by radiation pressure contributes significantly to the interstellar medium and thus to the chemical evolution of galaxies. All hot stars are soft x-ray sources; in contrast to the sun with its highly variable x-ray flux, long time scale x-ray variability is not common among hot stars. An analysis is presented here of an unusual increase in x-ray flux observed with the roentgen observatory satellite during a period of 2 days for the hot supergiant zeta Orionis, the only episode of x-ray variability that has been found in a hot star. These observations provide the most direct evidence so far for the scenario of shock-heated gas in the winds of hot stars.     

High-Pressure Polymorph of Thulium: An X-ray Diffraction Study

An x-ray diffractiotn study of thulium at room temperature and high pressure by means of a diamond-anvil press has shown that thulium transforms from a hexagonal close-packed structure to the samarium type, as other rareearth elements (gadolinium, terbium, dysprosium, and holmium) do. Unlike the other rare-earth elements, thulium (hexagonal close-packed) has an axial ratio (c/a) that is independent of pressure within experimental error and the transition is reversible. The transition occurs with increasing pressure in the range of 60 to 116 kilobars. The lattice paralieters of the samarium-type phase of thulium at about 116 kilobars are a = 3.327 +/- 0.005 angstroms and c = 23.48 +/- 0.04 angstroms, and the volume change at the transition is estimated to be - 0.5 percent of the volume of the hexagonal close-packed phase at the transition.     

A Benzene-Thermal Synthetic Route to Nanocrystalline GaN

A thermal reaction of Li3N and GaCl3 in which benzene was used as the solvent under pressure has been carried out for the preparation of 30-nanometer particles of gallium nitride (GaN) at 280°C. This temperature is much lower than that of traditional methods, and the yield of GaN reached 80%. The x-ray powder diffraction pattern indicated that sample was mainly hexagonal-phase GaN with a small fraction of rocksalt-phase GaN, which has a lattice constant a = 4.100 angstroms. This rocksalt structure, which had been observed previously only under high pressure (at least 37 gigapascals) was observed directly with high-resolution electron microscopy.     

X-ray-induced dissociation of H2O and formation of an O2-H2 alloy at high pressure

When subjected to high pressure and extensive x-radiation, water (H2O) molecules cleaved, forming O-O and H-H bonds. The oxygen (O) and hydrogen (H) framework in ice VII was converted into a molecular alloy of O2 and H2. X-ray diffraction, x-ray Raman scattering, and optical Raman spectroscopy demonstrated that this crystalline solid differs from previously known phases. It remained stable with respect to variations in pressure, temperature, and further x-ray and laser exposure, thus opening new possibilities for studying molecular interactions in the hydrogen-oxygen binary system.