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1.
The Hörnum tidal inlet is located in the German sector of the Wadden Sea between the barrier islands Sylt and Amrum. On the basis of seven bathymetric surveys of the tidal inlet covering a 55-yr period from 1939 to 1994 and long-term records from two tide gauges, a process-response analysis for the inlet was carried out. Following the method described by Dean and Walton (1975), the volume of sediment stored in the ebb–tidal delta (Vebd) was calculated for each survey. Furthermore, the cross-sectional area of the tidal inlet throat (Ac), and the area-height distribution (hypsographic curve) of the study area were established. Between 1939 and 1994, Ac increased by about 32%, whereas Vebd diminished by about 18%. Earlier investigations by, e.g., O'Brien (1931), Walton and Adams (1976), Dean (1988), and Eysink and Biegel (1992) documented a strong positive correlation between these two morphological parameters and tidal currents in the inlet. In conformity with the literature, regression of the values of Ac with the corresponding fall velocities of the water level in the inlet (a direct indication of ebb–tidal currents) produced a correlation coefficient (r) of 0.87. However, Vebd shows a weak negative correlation (r=−0.74) with fall velocities of the water level. Apparently, other factors than changes in ebb–tidal currents must have been responsible for the decrease in Vebd. It is suggested that one factor might be the observed considerable increase in storminess, i.e. wave action, in the region since approximately 1960. Consequently, storm wave action on the ebb–tidal delta intensified and erosion was here initiated. As a result, the swash bars of the ebb–tidal delta, in particular, suffered from severe erosion between 1959 and 1994. As the ebb–tidal currents increased simultaneously, sand supply to the terminal lobe of the ebb–tidal delta increased as well, thereby balancing the loss of sediment from the terminal lobe caused by the increasing storminess. The results of this case study indicate that the ratio of ebb–tidal currents to (storm) wave action determines, in large part, the value of Vebd.  相似文献   

2.
Smirnova  K. I.  Wiebe  D. S. 《Astronomy Reports》2019,63(6):445-459

The parameters of the radiation of interstellar matter in star-forming complexes in the high-metallicity galaxies NGC 628, NGC 2976, and NGC 3351, which have different morphological types, are analyzed. The relationship between the emission in Hα and in lines of CO and HI is considered, as well as the relationship between Hα and the emission of dust in the infrared range (IR). The fluxes and surface brightnesses in the UV and IR correlate well with the Hα emission. The HI emission also correlates well with Hα, while the correlation between the CO and Hα emission is much weaker. The ratio of the fluxes at 8 and 24 µm decreases with increasing Hα flux. This may be due to changes in the properties of the dust ensemble (a decrease in the mass fraction of polycyclic aromatic hydrocarbons) or to changing excitation conditions. Analysis of the kinematics of the CO lines shows that the CO flux grows with increasing velocity scatter ΔV when ΔV ? 70 km/s. Preliminary evidence for the existence of star-forming complexes with higher values of ΔV is provided, when the increase in the velocity scatter is accompanied by a decrease in the CO luminosity of the complex.

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3.
Vanadium occurs in multiple valence states in nature, whereas Nb is exclusively pentavalent. Both are compatible in rutile, but the relationship of V–Nb partitioning and dependence on oxygen fugacity (expressed as fO2) has not yet been systematically investigated. We acquired trace-element concentrations on rutile grains (n = 86) in nine eclogitic samples from the Dabie-Sulu orogenic belt by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and combined them with published results in order to assess the direct and indirect effects of oxygen fugacity on the partitioning of V and Nb into rutile. A well-defined negative correlation between Nb (7–1,200 ppm) and V concentrations (50–3,200 ppm) was found, documenting a competitive relationship in the rutile crystal that does not appear to be controlled by bulk rock or mineral compositions. Based on the published relationship of RtDV and V valence with ?QFM, we suggest that the priority order of V incorporation into rutile is V4+ > V3+ > V5+. The inferred Nb–V competitive relationship in rutile from the Dabie-Sulu orogenic belt could be explained by decreasing fO2 due to dehydration reactions involving loss of oxidizing fluids during continental subduction: The increased proportion of V3+ (expressed as V3+/∑V) and attendant decrease in RtDV is suggested to lead to an increase in rutile lattice sites available for Nb5+. A similar effect may be observed under more oxidizing conditions. When V5+/∑V increases, RtDV shows a dramatic decline and Nb concentration increases considerably. This is possibly documented by rutile in highly metasomatized and oxidized MARID-type (MARID: mica–amphibole–rutile–ilmenite–diopside) mantle xenoliths from the Kaapvaal craton, which also show a negative V–Nb covariation. In addition, their Nb/Ta covaries with V concentrations: For V concentrations <1,250 ppm, Nb/Ta ranges between 35 and 45, whereas for V > 1,250 ppm, Nb/Ta is considerably lower (5–15). This relationship is mainly controlled by a change in Nb concentrations, suggesting that the indirect dependence of RtDNb on fO2, which is not mirrored in RtDTa, can exert considerable influence on rutile Nb–Ta fractionation.  相似文献   

4.
构建了等直径不同裂隙密度和等裂隙密度不同裂隙直径两组物理模型,进行不同围压条件下多方向的超声波速度测试,并运用Hudson理论进行了理论模型计算。结果显示,计算与实测结果吻合较好。随围压的增大,纵、横波速度均近线性增加,纵、横波各向异性基本保持不变;裂隙密度从2%增大到6%,纵波速度不同程度降低,其中慢纵波降低幅度相对较大,快横波变化不明显,而慢横波则大幅降低。随着裂隙密度的增大,纵、横波各向异性均增大,且横波各向异性增加速率大于纵波;裂隙直径从2 mm增大到3 mm,快纵波速度增加很小,慢纵波增加明显,横波速度均不发生改变。随着裂隙直径的增大,纵波各向异性逐渐降低,横波各向异性保持不变。最后结合试验结果分析了Hudson理论在不同深度进行参数预测的必要条件。研究结果有助于油气生产、地下水的开采与控制、污染处理等。   相似文献   

5.

Shephards Discordant Zone is a 500–600 m thick interlayered sequence of deformed, altered and metamorphosed magnetite metagabbro and about 50 layers or lenses of magnetitite (> 80–90% magnetite). The sequence shows progressive magmatic fractionation upwards: Ti and Ti/Fe increase, and V, V/Ti and Cr decrease upwards in magnetite and in whole‐rock compositions. The main magnetite‐rich sequence (about 400 m thick) is deeply weathered, with 40 m of saprolite showing vertical zonation of weathering minerals due to progressive weathering. Magnetitites (average 1% V2O3) are resistant to weathering and show little chemical change, but magnetite gabbros (average 0.27% V2O3) are extensively weathered and show progressive loss of Ca, Na, Mg and S. Plagioclase, magnetite (1.37% V2O3), chlorite (up to 0.35% V2O3), actinolite, epidote and minor sulfides in unweathered rocks weather to kaolinite, hematite, goethite and minor vermiculite, ilmenite remaining largely unaffected. Vanadium is essentially immobile during weathering and is unaffected during weathering of magnetitites (1% V2O3), but is slightly depleted during weathering of magnetite gabbros (0.23% V2O3).  相似文献   

6.
The Bader topological analysis has been applied to ab initio computed electron densities of beryl, in order to clarify its mechanism of compression. Full structural optimization and total energy (E) calculations were performed at different cell volumes (V c). The pressure at each volume and the equation of state were estimated from the first and second derivatives of the resultant E(V c) curve. The total (negative) potential energy of the crystal, sum of both attractive and repulsive electrostatic terms, was found to systematically decrease (i.e., it moved to more negative values) up to the highest pressure considered (28.4 GPa), indicating that interelectronic and internuclear repulsions are not the only terms controlling the compressibility, at least in the pressure range investigated. Electronic kinetic energy increases as the cell volume is reduced, leading to a parallel increase of the total energy. Both structure at equilibrium and compressibility are therefore due to the balance between the opposing kinetic and potential energy terms. The Bader theory has been used to identify the topological atoms within the structure and to calculate their properties, with particular attention to the forces driving the structural relaxation at high pressure. On a qualitative basis, the obtained results are expected to be transferable to the discussion of compressibility of other mineral phases.  相似文献   

7.
A synthetic clinopyroxene with composition LiFe3+Ge2O6, monoclinic s.g. P21/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) Å, β = 108.844(6)°, V = 442.75(16) Å3, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P21/c to C2/c at a transition temperature T tr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below T tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > T tr the thermal volume expansion is due to dilatation of the structure in the $(\bar{1}\,0\,1) A synthetic clinopyroxene with composition LiFe3+Ge2O6, monoclinic s.g. P21/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) ?, β = 108.844(6)°, V = 442.75(16) ?3, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P21/c to C2/c at a transition temperature T tr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below T tr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > T tr the thermal volume expansion is due to dilatation of the structure in the ([`1] 0 1)(\bar{1}\,0\,1) plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β. From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P21/c expands more than the C2/c phase. The evolution of Q 2 (calculated as the normalized intensity of b-type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q 02 = 0.538(2) at T tr, with T c of 481(7) K, b/a = −2,290 K, and c/a = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe3+Si2O6) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P21/c ↔ C2/c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13.3%, compared to ca. 1.3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T > T tr (<1‰), nor a change of tilting of the basal plane. No change in coordination is observed at T > T tr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at T tr of the kinking angle (O3–O3–O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T < T tr [206.8(5)° at 703 K] and extends by 12° at the transition.  相似文献   

8.
We have measured the surface controlled dissolution rates of natural calcium carbonate minerals (limestone and marble) in H2O–CO2 solutions by using free drift batch experiments under closed system conditions with respect to CO2, at 10°C with an initial partial pressure of carbon dioxide of 5 · 10−2 atm. All experiments revealed reaction rates F, which can be described by the empirical relation: Fn1 = kn1 · (1 − c/ceq)n1 for c < cs, which switches to a higher order n2 for calcium concentrations c ≥ cs described by Fn2 = kn2 · (1 − c/ceq)n2. kn1 and kn2 are rate constants in mmole/(cm2 · s), ceq is the equilibrium concentration with respect to calcite. The values of the constants n1, n2, kn1, kn2 and cs depend on the V/A ratio employed, where V is the volume of the solution and A is the surface area of the reacting mineral. Different calcium carbonate minerals exhibit different values of the kinetic constants. But generally with increasing V/A, there is a steep variation in the values of all kinetic constants, such that the rates are reduced with increasing V/A ratio. Finally with sufficiently large V/A these values become constant. These results are explained by assuming intrinsic inhibitors in the bulk of the mineral. During dissolution these are released from the calcite matrix and are adsorbed irreversibly at the reacting surface, where they act as inhibitors. The thickness d of the mineral layer removed by dissolution is proportional to the V/A ratio. The amount of inhibitors released per surface area is given by d · cint, where cint is their concentration in the bulk of the mineral. At low thicknesses up to ≈3 · 10−4 cm in the investigated materials, the surface concentration of inhibitors increases until saturation is attained for thicknesses above this value.To analyze the surface concentration and the type of the inhibitors we have used Auger spectroscopy, which revealed the presence of aluminosilicate complexes at the surface of limestone, when a thickness of d ≈ 10−3 cm had been removed by dissolution. In unreacted samples similar signals, weaker by one order of magnitude, were observed. Depth profiles of the reacted sample obtained by Ar-ion sputtering showed the concentration of these complexes to decrease to the concentration observed in the unreacted sample within a depth of about 10 nm. No change of the concentration with depth was observed in unreacted samples. These data suggest that complexes of aluminosilicates act as inhibitors, although other impurities cannot be excluded.  相似文献   

9.
The compressional wave velocities (Vp), pressure derivatives (Vp′) and anisotropy (A) of three types of eclogites and country rocks from the Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt, China, have been measured under confining pressures up to 800 MPa. Type-1 eclogites, which are coarse-grained and subjected to almost no retrograde metamorphism, experienced recovery-accommodated dislocation creep at peak metamorphic conditions (in the diamond stability field). Type-2 eclogites are fine-grained reworked Type-1 materials that experienced recrystallization-accommodated dislocation creep under quartz/coesite boundary conditions during the early stage of exhumation. Type-3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust. Type-1 eclogites are richer in Al2O3 and MgO but poorer in SiO2 and Na2O than Type-2 and Type-3 eclogites. Anisotropy of Type-1 and Type-2 eclogites is generally low (<4%) because volumetrically important garnet is elastically quasi-isotropic, while Type-3 eclogites can exhibit high anisotropy (>10%) due to the presence of strongly anisotropic retrograde minerals such as amphibole, plagioclase and mica. The transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure (Pc), which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks. The Vp–pressure relationship can be expressed by Vp=a(lnP)2+blnP+c (P≤Pc) and Vp=V0+DP (P≥Pc), where P is the confining pressure, a and b are constants describing the closure of microcracks below Pc, c is the velocity when P is equal to one (MPa), V0 is the projected velocity of a crack-free sample at room pressure, and D is the intrinsic pressure derivative above Pc. When data are curve-fit, pressure derivatives and anisotropy as functions of pressure are determined. The average Vp of the eclogites in the linear regime is 8.42+1.41×10−4P for Type-1, 7.80+1.58×10−4P for Type-2, and 7.33+2.04×10−4P for Type-3, where Vp is in km/s and P in MPa. The decrease in V0 and increase in D from Type-1 to Type-3 eclogites are attributed to a decrease in garnet content and an increase in retrograde minerals. The NE–SW trending, NW-dipping, slab-like high Vp anomaly (8.72 km/s at a depth of 71 km) which extends from the Moho to at least 110 km beneath the Dabie–Sulu region, can be interpreted as the remnant of a subducted slab which is dominated by Type-1 eclogites and has frozen in the upper mantle since about 200–220 Ma. Such relic crustal materials, subducted and preserved as eclogite layers intercalated with felsic gneiss, garnet–jadeite quartzite, marble and serpentinized peridotite, could be responsible for regionally observed seismic reflectors in the upper mantle.  相似文献   

10.
Two synthetic single-crystals with composition Li(Al0.53Ga0.47)Si2O6 and LiGaSi2O6 and space group C2/c at room conditions have been studied under pressure by means of X-ray diffraction using a diamond anvil cell. The unit-cell parameters were determined at 12 and 10 different pressures up to P = 8.849 and P = 7.320 GPa for Li(Al0.53Ga0.47)Si2O6 and LiGaSi2O6, respectively. The sample with mixed composition shows a C2/c to P21/c phase transformation between 1.814 and 2.156 GPa, first-order in character. The transition is characterised by a large and discontinuous decrease in the unit-cell volume and by the appearance of the b-type reflections (h + k = odd) typical of the primitive symmetry. The Ga end-member shows the same C2/c to P21/c transformation at a pressure between 0.0001 and 0.39 GPa. The low-pressure value at which the transition occurred did not allow collecting any data in the C2/c pressure stability field except that on room pressure. Our results compared with those relative to spodumene (LiAlSi2O6, Arlt and Angel 2000a) indicate that the substitution of Al for Ga at the M1 site of Li-clinopyroxenes strongly affects the transition pressure causing a decrease from 3.17 GPa (spodumene) to less than 0.39 GPa (LiGaSi2O6) and decreases the volume discontinuity at the transition. As already found for other compounds, the C2/c low-pressure phases are more rigid than the P21 /c high-pressure ones. Moreover, the increase of the M1 cation radius causes a decrease in the bulk modulus K T0. The axial compressibility among the Li-bearing clinopyroxenes indicates that the c axis is the most rigid for the C2/c phases while it becomes the most compressible for the P21 /c phases.  相似文献   

11.
The unit-cell parameters of two columbite samples along the (Fe,Mn)Nb2O6 solid solution were measured by means of high-pressure single-crystal X-ray diffraction up to pressures of 7 GPa. The compressional behaviour of these minerals was studied as a function of composition and degree of order. The P–V data of all the samples were fitted with a third-order Birch–Murnaghan equation of state. For the two samples with different compositions but identical degree of order the substitution of Mn for Fe causes a decrease of the bulk modulus K T0, from 153(1) to 146(1) GPa, without any effect on the pressure first derivative K′. For the two samples with the same composition, cation ordering causes an increase of the bulk modulus from 149(1) to 153(1) GPa and of the pressure first derivative from 4.1(2) to 4.8(3). The compressional behaviour is anisotropic with a linear axial compressibility scheme β b > β c ≥ β a for all samples, regardless of composition and degree of order. Such anisotropy increases sligthly with increasing Mn content.  相似文献   

12.
 Synchrotron radiation S K-edge XANES spectra and unit-cell parameters are used to investigate the local electronic structure of non-stoichiometric binary and ternary Fe-Co-Ni monosulfide solid solution (mss; M0.923S, M = Fe, Co, Ni) quenched from 800 °C and low pressure. The prominent absorption edge feature of the XANES spectra represents transition of S 1s core level electrons to unoccupied S 3p σ* antibonding orbitals hybridized with empty metal 3d(eg) orbitals. There is a progressive increase in area of the edge peak from Fe0.923S to Ni0.923S and Co0.923S, which correlates with progressive decrease in c and a parameters for the NiAs-type subcell and increase in metallic character, and reflects increase in the number and availability of empty eg β orbitals and covalence of metal-S bonds. More generally, the area of the edge peak exhibits an inverse linear correlation with a, c and unit-cell volume of binary and ternary mss. This inverse linear correlation is attributed to progressive increase in covalence and M-S-M bonding interaction in the c-axis direction, through metal-S [M 3d(eg) - S 3p (or 3d)] π bonding. However, the area of the edge peak does not correlate very well with the average number of 3d electrons per metal atom in these solid solutions, showing that the absorption of synchrotron radiation reflects the local electronic structure of individual absorber atoms (i.e. the SM6 cluster), and is not a group (crystal energy band) effect. Received: 21 March 2000 / Accepted: 14 July 2000  相似文献   

13.
This study investigated the removal of triethylamine using a biotrickling filter. The influence of affecting parameters, such as height and recirculation liquid rate (V L) on contaminant removal efficiency, was examined in detail. The results demonstrated that in the constant empty bed residence time (EBRT), when V L was increased, the removal efficiency (RE) increased. Also, for a specific V L, increasing EBRT could also increase RE values. However, it seems that an increasing V L is a more cost-effective way to enhance RE as compared to an increasing EBRT. The obtained outcomes represented that for a constant EBRT, an increase in inlet loading (IL) could decrease RE. For lower ILs, the removal of the contaminant could be carried out faster in height. The first part of the bed contributed the most to contaminant removal, and for the lower ILs, the contribution was even further. For the first section of bed in a constant IL, increasing EBRT could increase RE. In a constant IL and EBRT, increasing V L could increase RE, as well as the removed mass loading by at least 20–25 g/m3/h. Also, the effect of the V L increase on removal amount in the second and third sections of the bed was negligible.  相似文献   

14.
Summary The laboratory testing program reported herein describes the statistical correlations of unconfined compressive strength ( c ), Young's modulus (E t50), and modulus ratio (MR) to their coefficients of variation (CV( c ), CV(E t50), and CV(MR)). Negative correlation of both c andE t50 to CV( c ) indicates that data scatter tends to decrease with increasing strength and stiffness. A positive correlation between MR and CV( c ) indicates that stiffer rocks are more variable for a given compressive strength.  相似文献   

15.
The adiabatic single-crystal elastic moduli of a natural sample of monticellite (CaMgSiO4), with the olivine crystal structure, have been measured under ambient conditions using Brillouin spectroscopy. From the single-crystal moduli the aggregate bulk and shear moduli are calculated to be K s=106±1 and = 55.2±0.4 GPa, respectively. These results are consistent with a systematic decrease in bulk modulus with increasing molar volume among the olivine-structured silicates. The longitudinal moduli decrease in the order c 11>c 33>c22, indicating that the structure is stiffest along the a axis and most compliant along the b axis. This relationship among the longitudinal moduli holds for all silicate and germanate olivines, and is thus inferred to result from the topology of the olivine crystal structure. However, the moduli obtained in this study are at variance with previous conclusions concerning deviations from the Cauchy relations (e.g. c 12=c 44). For monticellite, off-diagonal shear moduli of the c 12-type are uniformly greater than pure shear moduli such as c 44. Similar behavior is found in pyroxenes such as diopside. The relative magnitudes of shear and off-diagonal moduli are not, therefore, a diagnostic chemical signature in minerals with complex crystal structures.Department of Applied Physics, Stanford University, Stanford CA, USA  相似文献   

16.
Lamé modulus (λ) and shear modulus (μ) are among the most important,intrinsic,elastic constants of rocks.Using λ and μ could be much more advantageous than using P- and S-wave velocities (Vp and Vs).He...  相似文献   

17.
The structural evolution of tectonically deformed coals（TDC） with different deformational mechanisms and different deformational intensities are investigated in depth through X-ray diffraction（XRD） analysis on 31 samples of different metamorphic grades(Ro,max:0.7%-3.1%) collected from the Huaibei coalfield.The results indicated that there are different evolution characteristics between the ductile and brittle deformational coals with increasing of metamorphism and deformation.On the one hand,with the increase of metamorphism,the atomic plane spacing (d002) is decreasing at step velocity,the stacking of the BSU layer（Lc） is increasing at first and then decreasing,but the extension of the BSU layer（La） and the ratio of La/Lc are decreasing initially and then increasing.On the other hand,for the brittle deformational coal,d002 is increasing initially and then decreasing,which causes an inversion of the variation of Lc and La under the lower-middle or higher-middle metamorphism grade when the deformational intensity was increasing.In contrast,in the ductile deformational coals,d002 decreased initially and then increased,and the value of Lc decreased with the increase of deformational intensity.But the value of La increased under the lower-middle metamorphism grade and increased at first and then decreased under the higher-middle metamorphism grade.We conclude that the degradation and polycondensation of TDC macromolecular structure can be obviously impacted during the ductile deformational process,because the increase and accumulation of unit dislocation perhaps transforms the stress into strain energy.Meanwhile,the brittle deformation can transform the stress into frictional heat energy,and promote the metamorphism and degradation as well.It can be concluded that deformation is more important than metamorphism to the differential evolution of the ductile and brittle deformational coals.  相似文献   

18.
New high-pressure orthorhombic (GdFeO3-type) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) Å, c=7.690 (8) Å and V= 221.99 (15) Å3. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) Å, b=5.169 (5) Å, c=7.239 (9) Å and V= 187.94 (36) Å3. Based on published data on the quench phases, the FeTiO3 perovskite breaks down to a rocksalt + baddelyite mixture of FeO and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.  相似文献   

19.
A high-pressure single-crystal x-ray diffraction study of perovskite-type MgSiO3 has been completed to 12.6 GPa. The compressibility of MgSiO3 perovskite is anisotropic with b approximately 23% less compressible than a or c which have similar compressibilities. The observed unit cell compression gives a bulk modulus of 254 GPa using a Birch-Murnaghan equation of state with K set equal to 4 and V/V 0 at room pressure equal to one. Between room pressure and 5 GPa, the primary response of the structure to pressure is compression of the Mg-O and Si-O bonds. Above 5 GPa, the SiO6 octahedra tilt, particularly in the [bc]-plane. The distortion of the MgO12 site increases under compression. The variation of the O(2)-O(2)-O(2) angles and bondlength distortion of the MgO12 site with pressure in MgSiO3 perovskite follow trends observed in GdFeO3type perovskites with increasing distortion. Such trends might be useful for predicting distortions in GdFeO3-type perovskites as a function of pressure.  相似文献   

20.
High-pressure single-crystal X-ray diffraction measurements of lattice parameters of the compound Li2VOSiO4, which crystallises with a natisite-type structure, has been carried out to a pressure of 8.54(5) GPa at room temperature. Unit-cell volume data were fitted with a second-order Birch-Murnaghan EoS (BM-EoS), simultaneously refining V 0 and K 0 using the data weighted by the uncertainties in V. The bulk modulus is K 0 = 99(1) GPa, with K′ fixed to 4. Refinements of third order equations-of-state yielded values of K′ that did not differ significantly from 4. The compressibility of the unit-cell is strongly anisotropic with the c axis (K 0(c) = 49.7 ± 0.5 GPa) approximately four times more compressible than the a axis (K 0(a) = 195 ± 3 GPa).  相似文献   

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