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M. Colella G. R. Lumpkin Z. Zhang E. C. Buck K. L. Smith 《Physics and Chemistry of Minerals》2005,32(1):52-64
In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO2, CaUO4, SrCa2UO6, UTi2O6, and Y0.5U0.5Ti2O6) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M5/(M4 +M5), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimise the effects of electron beam reduction of U6+ and U5+. Test samples consisted of three additional synthetic brannerites (Th0.7U0.3Ti2O6, Ca0.2U0.8Ti2O6, and Th0.55U0.3Ca0.15Ti2O6) and three natural brannerites from different localities. The natural brannerite samples are all completely amorphous, due to cumulative alpha decay events over geological time periods (24–508 Ma). Our U valence calibration results are in reasonable agreement with previous work, suggesting possibly a non-linear relationship between the branching ratio and the number of f electrons (and hence the average valence state) of U in solids. We found excellent agreement between the nominal valence states of U and the average valence states determined directly by EELS and estimated by EDX analysis (with assumptions regarding stoichiometry) in two of the three synthetic brannerite test samples. The average U oxidation states of the five synthetic brannerite samples, as derived from XPS analyses, are also in good agreement with those determined by other techniques. The average valence states of U in three amorphous (metamict) natural brannerite samples with alpha decay doses ranging from 3.6×1016 to 6.9×1017 /mg were found to be 4.4, 4.7, and 4.8, consistent with the presence of U5+ and/or U6+ as well as U4+ in these samples. These results are in general agreement with previous wet chemical analyses of natural brannerite. However, the average valence states inferred by SEM-EDX for two of the natural brannerite samples do not show satisfactory agreement with the EELS determined valence. This may be due to the occurrence of OH– groups, cation vacancies, anion vacancies, or excess oxygen in the radiation-damaged structure of natural brannerite. 相似文献
3.
High-resolution manganese K-edge X-ray absorption near edge structure spectra were collected on a set of 40 Mn-bearing minerals.
The pre-edge feature information (position, area) was investigated to extract as much as possible quantitative valence and
symmetry information for manganese in various “test” and “unknown” minerals and glasses. The samples present a range of manganese
symmetry environments (tetrahedral, square planar, octahedral, and cubic) and valences (II to VII). The extraction of the
pre-edge information is based on a previous multiple scattering and multiplet calculations for model compounds. Using the
method described in this study, a robust estimation of the manganese valence could be obtained from the pre-edge region at
5% accuracy level. This method applied to 20 “test” compounds (such as hausmannite and rancieite) and to 15 “unknown” compounds
(such as axinite and birnessite) provides a quantitative estimate of the average valence of manganese in complex minerals
and silicate glasses.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
4.
铀矿物和含铀矿石中铀价态比值的X射线光电子能谱分析 总被引：1，自引：0，他引：1
在应用Ｘ射线光电子能谱对铀氧化物中铀价态比值定量分析的基础上，研究了对铀矿物和含铀矿石中铀价态的测量方法。结果表明，从含铀量０．９％－７２％的样品测得的Ｕ４＋／Ｕ总的比值与化学方法的结果相符。其分析误差能满足铀矿地质研究的需要。 相似文献
5.
H. W. Nesbitt A. G. Schaufuss G. M. Bancroft R. Szargan 《Physics and Chemistry of Minerals》2002,29(1):72-77
Synchrotron excited X-ray photoelectron spectra (SXPS) of hexagonal pyrrhotite reveal three distinct Fe 3d-derived photopeaks within its outer valence band. The t
2gα band (majority spin) is centered at about 2.5 eV, the e
g
α band at about 1.0 eV and the t
2gβ (minority spin) contribution at about 0.25 eV. From these data the ligand field splitting energy is 1.5 (±0.2) eV and the
majority spin pairing energy is 2.25 (±0.2) eV. These are the first such XPS measurements for this mineral. S 3p-derived bonding and non-bonding bands are identified, with the former centred at about 6.5 eV and the latter near 4.5 eV.
The XPS results are remarkably consistent with SCF-Xα scattered wave molecular orbital calculations. Although the calculations
and the collected spectra are consistent, they differ from a recent interpretation of the pyrrhotite valence band. An explanation
for the discrepant results is provided. Auger resonant enhancement of Fe 3d photopeaks at 60 eV photon energy results in the
t
2gα emission (at 2.5 eV) being strongly enhanced and broader than the t
2gβ emission (0.25 eV). The explanation of these observations requires the presence of weak Fe–Fe π and π* crystal (molecular) orbitals located near 2.5 eV, and separated by no more than about 0.5 eV. The π-bonded crystal orbitals
are derived from weak mixing of adjacent Fe t
2g atomic orbitals along the c crystallographic axis.
Received: 15 June 2000 / Accepted: 11 June 2001 相似文献
6.
J. Li W. Sturhahn J. M. Jackson V. V. Struzhkin J. F. Lin J. Zhao H. K. Mao G. Shen 《Physics and Chemistry of Minerals》2006,33(8-9):575-585
We investigated the valence state and spin state of iron in an Al-bearing ferromagnesian silicate perovskite sample with the composition (Mg0.88Fe0.09)(Si0.94Al0.10)O3 between 1 bar and 100 GPa and at 300 K, using diamond cells and synchrotron Mössbauer spectroscopy techniques. At pressures below 12 GPa, our Mössbauer spectra can be sufficiently fitted by a “two-doublet” model, which assumes one ferrous Fe2+-like site and one ferric Fe3+-like site with distinct hyperfine parameters. The simplest interpretation that is consistent with both the Mössbauer data and previous X-ray emission data on the same sample is that the Fe2+-like site is high-spin Fe2+, and the Fe3+-like site is high-spin Fe3+. At 12 GPa and higher pressures, a “three-doublet” model is necessary and sufficient to fit the Mössbauer spectra. This model assumes two Fe2+-like sites and one Fe3+-like site distinguished by their hyperfine parameters. Between 12 and 20 GPa, the fraction of the Fe3+-like site, Fe3+/∑Fe, changes abruptly from about 50 to 70%, possibly due to a spin crossover in six-coordinate Fe2+. At pressures above 20 GPa, the fractions of all three sites remain unchanged to the highest pressure, indicating a fixed valence state of iron within this pressure range. From 20 to 100 GPa, the isomer shift between the Fe3+-like and Fe2+-like sites increases slightly, while the values and widths of the quadruple splitting of all three sites remain essentially constant. In conjunction with the previous X-ray emission data, the Mössbauer data suggest that Fe2+ alone, or concurrently with Fe3+, undergoes pressure-induced spin crossover between 20 and 100 GPa. 相似文献
7.
《Chemie der Erde / Geochemistry》2014,74(3):497-505
Chromium L3,2 energy loss near-edge structures (ELNES) of nine chromium compounds were measured in a transmission electron microscope to study the effect of valence state and site geometry of chromium on spectral features. The compounds studied represent a set of standards with given valence state and coordination. The chromium coordination is either octahedral or tetrahedral with different degree of distortion. The distortion of polyhedra results in a loss of the fine detail of spectral features in Cr L3,2 ELNES spectra. The effect of valence state manifests in a systematic chemical shift of the ELNES spectra by about 2 eV per oxidation state for octahedrally coordinated Cr and 0.4–1.4 eV per oxidation state for tetrahedrally coordinated Cr. On the basis of collected Cr L3,2 ELNES spectra we propose a simple quantification technique for the determination of chromium oxidation state in oxidic compounds which is independent of the coordination. 相似文献
8.
The charge density and bond character of the rutile-type structure of SiO2 (stishovite) under compression to 30 GPa were investigated by X-ray diffraction study using synchrotron radiation and AgKα rotating anode X-ray generator through a newly devised diamond-anvil cell. The valence electron density was determined by
least-squares refinement including the κ parameter and the electron population in the X-ray atomic scattering parameters.
The oxygen κ-parameter of SiO2 is 0.94 under ambient conditions and 1.11 at 29.1 GPa and the silicon valence changes from +2.12(8) at ambient pressure to
+2.26(15) at 29.1 GPa. These values indicate that the electron distributions are more localized with increasing pressure.
The difference Fourier map shows the deformation of the valence electron distribution and the bonding electron population
in residual electron densities. The bonding electron observed from the X-ray diffraction study is interpreted by molecular
orbital calculations. The deformation of SiO6octahedra and the bonding electron density of stishovite structures are elucidated from the overlapping electron orbits. The
O–O distances of shared and unshared edge of SiO6 octahedra change with the cation ionicity. The repulsive force between the two cations in the adjacent octahedron makes its
shared edge shorter. The pressure changes of the apical and equatorial Si–O interatomic distances are explained by the electron
density of state (DOS) of Si and electron configuration.
Received: 7 January 2002 / Accepted: 6 May 2002 相似文献
9.
山东蓝色蓝宝石的着色是由Ｔｉ４＋离子与Ｆｅ２＋离子合理配比而成，一般控制在Ｔｉ４＋／Ｆｅ２＋＝１的配比范围内。多余的Ｆｅ２＋和其它Ｆｅ３＋离子存在都不利于蓝色蓝宝石正常呈色。通过穆斯堡尔谱和电子顺磁共振谱分析，测定了山东蓝宝石中铁离子的不同价态和浓度。对Ｆｅ２＋和Ｆｅ３＋离子所引起的吸收光谱特征作了研究，查明只有Ｆｅ３＋浓度很高时才产生４７７ｎｍ吸收峰，是致使山东蓝宝石表观上呈深蓝带黑色调的的重要原因。 相似文献
10.
The effectiveness of CO2 storage in deep saline aquifers and hydrocarbon reservoirs is governed, among other factors, by the interfacial tension between the injected CO2 and formation water (brine). Experimental data on CO2/water and CO2/NaCl solution have revealed that the interfacial tension depends on the pressure, temperature and water salinity. However, there is still a lack of data for other salts (such as MgCl2 and CaCl2) which are also present in aquifers and carbonate reservoirs. 相似文献