Precision Lattice-Parameter Determination of (Mg,Fe)SiO3 Tetragonal Garnets

The tetragonal garnet (Mg,Fe)SiO(3) is a high-pressure phase of pyroxene that is thought to be a major constituent of the earth's upper mantle. Its crystal structure is similar to that of cubic garnet, but it is slightly distorted to tetragonal symmetry so that its x-ray powder diffraction pattern shows a very small line splitting. A suite of tetragonal garnets with different compositions in the MgSiO(3)-rich portion of the MgSiO(3)-FeSiO(3) system was synthesized at about 20 gigapascals and 2000 degrees C. The lattice parameters a and c of quenched samples were determined by whole-powder-pattern decomposition analysis of Fe Kalpha x-ray powder diffraction data, which has the capacity to resolve to a high degree heavily overlapping reflections. It was found that the lattice parameters can be obtained from the following equations; a (in angstroms) = 11.516 + 0.088x and c (in angstroms) = 11.428 + 0.157x, where x, teh mole fraction of FeSiO(3), is 0.0 相似文献   

Twinning in MgSiO3 Perovskite

Crystals of MgSiO(3) perovskite synthesized at high pressures and temperatures have orthorhombic symmetry under ambient conditions. Examination by transmission electron microscopy shows that the microstructure of crystals synthesized at 26 gigapascals and 1600 degrees C is dominated by a large number of twin domains that are related by reflection operations with respect to {112} and {110} planes. These twins may be associated with the transformations of MgSiO(3) perovskite from the cubic to tetragonal and tetragonal to orthorhombic phases, respectively, upon decreasing pressure and temperature. These observations suggest that under the experimental synthesis conditions, and perhaps in the earth's lower mantle, the stable phase of MgSiO(3) might have the cubic perovskite structure.     

Soil as a sink for atmospheric carbon monoxide

The rate of carbon monoxide oxidation by soil increased with increasing carbon monoxide concentration in the gas phase, in line with Michaelis-Menten kinetics. Rates of carbon monoxide oxidation were determined for 20 soils at 0 degrees , 10 degrees , 20 degrees , and 30 degrees C. The observed oxidation rates were used to calculate a global soil uptake rate of atmospheric carbon monoxide of 4.1 x 10(14) grams per year, which is slightly less than the amount of carbon monoxide believed to be produced annually as a result of fossil fuel combustion.     

Cubic FeS, a Metastable Iron Sulfide

Studies of the corrosion products of metallic iron formed in the absence of air oxidation in solutions of hydrogen sulfide have revealed the existence of a new phase, cubic FeS, associated with tetragonal iron sulfide, Fe(1 + x)S. This new phase is metastable and has a sphalerite-like structure.     

Xenon hexafluoride: structural crystallography of tetrameric phases

Three crystalline phases of xenon hexafluoride are based on tetrameric association of XeF(5)(+) and F(-) ions into eight-membered rings. Phase I (monoclinic, 8 XeF(6) units per cell) transforms at approximately 10 degrees C to phase II (orthorhombic, 16 XeF(6) units per cell), which in turn transforms at approximately -25 degrees C to phase 111 (monoclinic, 64 XeF(6) units per doubly primitive cell). The transformation from phase I to phase II requires gross reorientation of half of the tetramers in the structure. The transformation from phase II to phase III involves only an ordering of right-handed and left-handed configurations.     

Reaction of plutonium dioxide with water: formation and properties of PuO(2+x)

Results show that PuO(2+x), a high-composition (x 相似文献   

Crystal Structure of the High-Pressure Phase of Solid CO2

X-ray diffraction study of solid CO(2) at room temperature has shown that the powder pattern of the high-pressure phase, which supersedes the low-pressure cubic Pa3 phase at about 10 gigapascals, is consistently interpreted in terms of an orthorhombic Cmca structure. The orthorhombic cell at 11.8 gigapascals has dimensions of 4.330 +/- 0.015, 4.657 +/- 0.005, 5.963 +/- 0.009 angstroms for its a, b, and c faces, respectively, and a volume of 120.3 +/- 0.5 cubic angstroms. Four molecules contained in the unit cell are located at the base-centered positions with their molecular axes inclined at about 52 degrees with respect to the crystallographic c axis. The volume change associated with the Pa3-Cmca transition is close to zero. The structural dimensions obtained for the high-pressure crystalline phase of CO(2) are of great importance for a theoretical understanding of the role of intermolecular interactions, including quadrupole-quadrupole interactions, in molecular condensation.     

Diffusion creep in perovskite: implications for the rheology of the lower mantle

High-temperature creep experiments on polycrystalline perovskite (CaTiO(3)), an analog of (Mg,Fe)SiO(3) perovskite of the lower mantle, suggest that (grain size-sensitive) diffusion creep is important in the lower mantle and show that creep rate is enhanced by the transformation from the orthorhombic to the tetragonal structure. These observations suggest that grain-size reduction after a subducting slab passes through the 670-kilometer discontinuity or after a phase transformation from orthorhombic to tetragonal in perovskite will result in rheological softening in the top portions of the lower mantle.     

Heterogeneous catalysis: effect of an alternating electric field

Oxidation of carbon monoxide was studcied in a static reactor at 123 degrees +/- 1 degrees C, wtith NiO as catalyst. An alternating electric field of about 22,000 volts per centimeter, peak to peak, was applied normal to the catalytic Surfaces. The rate of oxidation was thereby enhanced by a factor of 6 at 100 cycles per second, with maximum enhancement at between 100 and 200 cycles per second.     

X-ray diffaction evidence for nonstoichiometric rubidium-c60 intercalation compounds

Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions chi in Rb(chi)C(60) is reported. In addition to the face-centered cubic (fcc) (chi = 3, superconducting), body-centered tetragonal (chi = 4), and body-centered cubic (bcc) (chi = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c 相似文献   

Mariner 9 celestial mechanics experiment: gravity field and pole direction of Mars

Analysis of the Mariner 9 radio-tracking data shows that the Martian gravity field is rougher than that of Earth or the moon, and that the accepted direction of Mars's rotation axis is in error by about 0.5 degrees . The new value for the pole direction for the epoch 1971.9, referred to the mean equatorial system of 1950.0, is right ascension alpha= 317.3 degrees +/- 0.3 degrees , declination delta = 52.6 degrees +/- 0.2 degrees . The values found for the coefficients of the low-order harmonics of Mars's gravity field are as follows: J(2)=(1.96+/-0.01)x10(-3), referred to an equatorial radius of 3394 kilometers; C(22) = -(5 +/- 1) x 10(-5); and S(22) = (3 +/- 1) x 10(-5). The value for J(2) is in excellent agreement with the result from, Wilkins' analysis of the observations of Phobos. The other two coefficients imply a value of (2.5 +/- 0.5) x 10(-4) for the fractional difference in the principal equatorial moments of inertia; the axis of the minimum moment passes near 105 degrees W.     

Shock-wave compression and x-ray studies of titanium dioxide

The Hugoniot of the rutile phase of titanium dioxide has been determined to 1.25 megabars, and data show the existence of a phase change at about 0.33 megabar. The volume decrease associated with this transformation appears to be quite large (approximately 21 percent). Rutile, when recovered from shockloading in excess of the transformation pressure, is found to be irreversibly transformed to the orthorhombic lead dioxide structure (a distortion of the fluorite structure) with parameters a, 4.529; b, 5.464; and c, 4.905 angstroms and a calculated density of 4.374 grams per cubic centimeter. The new phase reverts to rutile at temperatures above 450 degrees C. It is suggested that the new phase may be another diagnostic indicator of meteorite impact on the earth's surface.     

Manganese oxide octahedral molecular sieves: preparation, characterization, and applications

A thermally stable 3 x 3 octahedral molecular sieve corresponding to natural todorokite (OMS-1) has been synthesized by autoclaving layer-structure manganese oxides, which are prepared by reactions of MnO(4)(-) and Mn(2+) under markedly alkaline conditions. The nature and thermal stability of products depend strongly on preparation parameters, such as the MnO(4)(-)/Mn(2+) ratio, pH, aging, and autoclave conditions. The purest and the most thermally stable todorokite is obtained at a ratio of 0.30 to 0.40. Autoclave treatments at about 150 degrees to 180 degrees C for more than 2 days yield OMS-1, which is as thermally stable (500 degrees C) as natural todorokite minerals. Adsorption data give a tunnel size of 6.9 angstroms and an increase of cyclohexane or carbon tetrachloride uptake with dehydration temperature up to 500 degrees C. At 600 degrees C, the tunnel structure collapses. Both Lewis and Br?nsted acid sites have been observed in OMS-1. Particular applications of these materials include adsorption, electrochemical sensors, and oxidation catalysis.     

Mobile point defects and atomic basis for structural transformations of a crystal surface

Structural transformations on elemental semiconductor surfaces typically occur above several hundred degrees Celsius, and the atomic motions involved are extremely rapid and difficult to observe. However, on the (111) surface of germanium, a few lead atoms catalyze atomic motions so that they can be observed with a tunneling microscope at temperatures below 80 degrees C. Mass transport and structural changes are caused by the creation and propagation of both vacancy-like and interstitial-like point defects within the crystal surface. The availability of dangling bonds on the surface is critical. A detailed atomic model for the observed motions has been developed and is used to explain the structural phase transition Ge(111)-c(2x8) <--> 1x1, which occurs near 300 degrees C.     

The charnockite geotherm

Charnockite, a hypersthene-bearing granite, and other associated rocks of the charnockite series have a global distribution. These rocks, according to evidence from mineral-chemical and experimental phase equilibrium relations, formed or recrystallized at temperatures between 800 degrees and 900 degrees C and at relatively shallow depths of 6 to 12 kilometers. This evidence indicates the existence of geothermal gradients of 70 degrees to 100 degrees C per kilometer probably at various times, the latest being around 1300 x 10(6) years ago.     

Possible Evidence for Superconducting Layers in Single Crystal YBa2Cu3O7-x by Field Ion Microscopy

The high-transition-temperature superconducting ceramic material YBa(2)Cu(3)O(7-x) (0< x < 0.5) has been examined by field ion microscopy. Specimens from nominally superconducting and nonsuperconducting samples(determined by magnetic susceptibility measurements) were studied by field ion microscopy and significant differences were found. Preferential imaging of atomic or molecular layers, due to preferential field evaporation, field ionization, or both, was found in the superconducting phase below the transition temperature and is interpreted as possible evidence for the occurrence of relatively highly conducting layers in the YBa(2)Cu(3)O(7-x) unit cell perpendicular to the orthorhombic c-axis. Similar results were obtained for YbBa(2)Cu(3)(7-x).     

Quenchable high-pressure polymorph of zinc selenate

A new quenchable high-pressure form of zinc selenate (ZnSeO(4)) was produced by subjecting the low-pressure modification to 40 kilobars at 400 degrees C for 30 minutes. The new form is orthorhombic, space group D(2h),(17)-Cmcm. The cell constants at 29 degrees C are: a, 5.511 angstroms; b, 8.110 angstroms; and c, 6.585 angstroms. The calculated density is 4.70 grams per cubic centimeter in comparison with 4.61 grams per cubic centimeter for the low-pressure modification. This implies a volume change of 2 percent at the transition.     

Phase Transformation at High Temperatures in Hafnia and Zirconia

Phase transformation curves for HfO(2) and ZrO(2) have been made. Transformation hysteresis is discussed. The transformation of monoclinic to tetragonal as temperature increases occurs over the range 1000 degrees to 1200 degrees C in ZrO(2), and 1500 degrees to 1600 degrees C in HfO(2). With decreasing temperature the transition back to monoclinic occurs from 970 degrees to 750 degrees C in ZrO(2) and 1550 degrees to 1450 degrees C in HfO(2). These transformations have been visually observed in ZrO(2), but not in HfO(2).     

Hemoglobin Kinetics of the Galapagos Rift Vent Tube Worm Riftia pachyptila Jones (Pogonophora; Vestimentifera)

Kinetics of the reactions of Riftia pachyptila hemoglobin with oxygen were followed spectrophotometrically by stopped-flow and laser flash photolysis techniques. The rate of oxygen dissociation increases eightfold over the range of 5 degrees to 20 degrees C (k = 2.2 sec(-1)at 10 degrees C). Oxygen recombination after flash photolysis was biphasic. The rates of both slow and fast phases of the reaction were independent of temperature from 0 degrees to 20 degrees C(k'fast = 7 x 10(6); k'slow = 1 x 16(6) liter mole (-1) sec(-1)). As the oxygen affinity is relatively temperature independent, analysis in terms of the two-state model of cooperativity requires that the conformational equilibrium constant L decrease by about 50-fold between 3 degrees and 15 degrees C.     

Luminescence and thermoluminescence induced by bombardment with protons of 159 million electron volts

We have observed a red luminescence, qualitatively similar to that of enstatite achondrites, in unsorted fines, their separated mineral phases, and rock chips. The energy efficiency of the plagioclase fraction is approximately 1 percent. At -196 degrees C the effect is enhanced by a factor between 1 and 2. All fractions except ilmenite exhibit blue thermoluminescence with a glow peak near -135 degrees C and an energy efficiency approximately 4 x 10(-6). Unlike the thermoluminescence of terrestrial and meteoritic material, it is nonrepeatable even after annealing at 200 degrees C. Similar thermoluminescence is seen in rock chips, but in unsorted fines it is masked by the opaque fractions.