Structural states of Mg-cordierite II: Landau theory

Mg-cordierite undergoes a ferroelestic phase transitionP6/mmc-Cccm. The order parameterQ is proportional to the spontaneous strain as reflected by changes of the lattice parametersa andb during the phase transition. The order parameter,Q _od, which describes the Al, Si ordering, isnot directly involved in the phase transition and only “triggers” the structural collapse. Landau theory predicts that cordierite can exist in stable or metastable states with hexagonal, orthorhombic or monoclinic symmetry. Hexagonal cordierite can develop modulated structures which have been found by Putnis et al. (1987). The phase transition is predicted to be accompanied by singularities of the elastic constantsC ₁₁,C ₂₂ andC ₁₂ leading to an elastic softening of the crystal structure.     

Interpretation of heterogeneity effects in synchrotron X-ray fluorescence microprobe data

Heterogeneity effects often limit the accuracy of synchrotron X-ray fluorescence microprobe elemental analysis data to ± 30%. The difference in matrix mass absorption at Kα and Kβ fluorescence energies of a particular element can be exploited to yield information on the average depth-position of the element or account for heterogeneity effects. Using this technique, the heterogeneous distribution of Cu in a simple layered sample could be resolved to a 2 × 2 × 10 (x, y, z, where z is the depth coordinate) micrometer scale; a depth-resolution limit was determined for the first transition metal series and several other elements in calcite and iron oxide matrices. For complex heterogeneous systems, determination of average element depth may be computationally limited but the influence of heterogeneity on fluorescence data may still be assessed. We used this method to compare solid-state diffusion with sample heterogeneity across the Ni-serpentine/calcite boundary of a rock from Panoche Creek, California. We previously reported that Ni fluorescence data may indicate solid state diffusion; in fact, sample heterogeneity in the depth dimension can also explain the Ni fluorescence data. Depth heterogeneity in samples can lead to misinterpretation of synchrotron X-ray microprobe results unless care is taken to account for the influence of heterogeneity on fluorescence data.     

Imaging the heterogeneity of mineral surface reactivity using Ag(I) and synchrotron X-ray microscopy

Microscopic-scale imaging of reduced zones on the surfaces of minerals can be achieved by reaction with dilute Ag(I) solutions and subsequent analysis using synchrotron X-ray microscopy (XRM) above the Ag K-edge (25.5 keV). The principal reductant is Fe(II), but other reductants such as sulfide may contribute. Reduced zones may exist instrinsically, as in the structure of biotite and augite, or may be generated by reaction with chemical agents such as dithionite or treatment with sulfate-reducing bacteria (SRB). We demonstrate the method on flakes of specular hematite and biotite, as well as on thin sections of different rocks (arfvedsonitic granite, oolitic hematite, diabase, and quartz conglomerate) treated with SRB, and discuss possible artifacts that can occur. To our knowledge, this is the only microscopic technique that can image Fe(II) zones on the surface of an Fe-bearing mineral with monolayer sensitivity.     

Scolecite, Part I: Refinement of high-order data, separation of internal and external vibrational amplitudes from displacement parameters

A single crystal of scolecite, CaAl₂Si₃O₁₀· 3H₂O, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoKα radiation (λ=0.71069?Å) in a complete sphere of reflection up to sinθ/λ=0.9?Å^?1. The structure was refined in the pseudo-orthorhombic setting of space group F1d1 instead of the conventional setting Cc for better comparison with natrolite (Fdd2). The cell parameters are: a=18.502(1)?Å, b=18.974(2)?Å, c=6.525(1)?Å, β=90.615(7)°, V=2290.6(3)?ų, Z=8. A refinement of high-order diffraction data yielded residuals of R(F)=0.9%, R _w (F)=0.9%, GoF=1.73 for 1831 high-angle reflections (0.7≤sinθ/λ≤0.9?Å^?1) and R(F)=1.2%, R _w (F)=1.4%, GoF=3.22 for all 3478 independent reflections. In comparison with natrolite, the replacement of 2 Na⁺ by 1 Ca²⁺ and 1 H₂O leads to a reduction of symmetry from Fdd2 to F1d1. Each general atomic position in natrolite (except of Na) splits into two crystallographically independent positions in scolecite. The T?O distances and T?O?T angles of these two sites differ distinctly from each other due to the influence of the calcium ions on the framework. An unexpected result of our detailed analysis of the data is that the additional water molecule (O7) disturbs the symmetry of the framework to a greater extent than the replacement of Na⁺ by Ca²⁺. As a comparison of the displacement parameters indicates, the bonds within the tetrahedral framework and to the extraframework cations are stronger in scolecite than in natrolite. The isotropic U(equ) values of the framework atoms and extraframework cations are about 10% smaller in scolecite compared to natrolite. The same tendency is shown by the analysis of the internal vibrational amplitudes ΔU. The corresponding force constants are in the range of F=358 to 3367?Nm^?1 for the T?O bonds in scolecite (in natrolite: F=354 to 824?Nm^?1). The values of the force constants which determine the vibrations of the Ca ions and water molecules against the framework oxygen atoms lie in the range of F=33 to 1757?Nm^?1 (in natrolite: F=57 to 293?Nm^?1).     

Natrolite,part I: Refinement of high-order data,separation of internal and external vibrational amplitudes from displacement parameters

A single crystal of natrolite, Na₂Al₂Si₃O₁₀·2H₂O, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoKα radiation (λ = 0.7107 Å) in a complete sphere of reflection up to sin θ/λ = 0.903 Å^?1. The structure was refined in the orthorhombic space group Fdd2 with a = 18.2929 (7) Å, b = 18.6407(9) Å, c = 6.5871(6) Å, V = 2246 ų, Z = 8. A refinement of high-order diffraction data yielded reliability factors of R(F) = 0.9%, R _w(F) = 0.8%, GoF = 1.40 for 1856 high-angle reflections (0.7 ?in θ/λ <0.903 Å^?1) and R(F) = 1.0%, R _W(F) = 1.2%, GoF = 3.07 for all 3471 independent reflections in the complete sphere of reflection. The T-O distances as well as the T-O-T angles were found to be strongly influenced by the different bond strengths received by the individual oxygen atoms. The T O distances calculated using Baur's extended valence rule agree on average within 0.003 Å with the observed values. An analysis of the mean square displacement amplitudes allowed a separation of the external and internal vibrational amplitudes along the T-O bonds as well as along the Na O and H₂O-O bond directions and the calculation of force constants. The internal vibrational amplitudes (ΔU) of the T-O vibrations are in the range of 5 to 11 × 10^-4 Ų, that is about one order of magnitude smaller than the mean square displacement amplitudes of the external vibrations. The corresponding force constants are F = 354 to 824 Nm^?1. The values of the force constants of the motion of the Na-ion and the water molecule against the framework oxygen atoms lie in the range between F = 57 and 293 Nm^?1. This is the first instance where displacement amplitudes from a zeolite structure refinement could be apportioned between contributions from internal and external vibrations for individual bonds.     

Supersoft X-ray sources: Basic parameters

The parameters of ten supersoft X-ray sources observed by ROSAT obtained using blanketing approximations and LTE model atmospheres are analyzed. The consistency of the resulting parameters with a model with stable/recurrent burning on the surface of the white dwarf is studied. The luminosity and sizes of seven of the sources are in good agreement with this model. The masses of the white dwarfs in these sources are estimated. A formula that can be used to estimate the masses of white dwarfs in classical supersoft sources based on their effective temperatures is presented.     

碱性玄武岩中长石巨晶的结构水：红外光谱和核磁共振谱研究

运用傅立叶变换红外光谱技术（ＦＴＩＲ,红外区和近红外区）和质子魔角旋转核磁共振技术（＾１ＨＭＡＳＮＭＲ）对来自安徽女山,江苏盘石山和河北汉诺坝新生代大恶性玄武岩中的４个歪长石巨晶进行了观察,结果显示,这些巨晶含有结构水,主要以Ｈ２Ｏ的形式存在,其中３个样品的结构水含量（Ｈ２Ｏ）分别为４０５μｇ／ｇ,９１５μｇ／ｇ,μｇ／ｇ。这些数据和文献中已有的资源表明,名义上无水的长石族矿物可以是地球上的水储库     

Structural and vibrational behaviour of fluorapatite with pressure. Part I: in situ single-crystal X-ray diffraction investigation

Using single-crystal X-ray diffraction from a diamond anvil cell, the compressibility of a synthetic fluorapatite was determined up to about 7?GPa. The compression pattern was anisotropic, with greater change along a than c. Unit cell parameters varied linearly with β _a =3.32(8)?10^?3 and β _c =2.40(5)?10^?3 GPa^?1, giving a ratio β _a :β _c =1.38:1. Data fitted with a third-order Birch-Murnaghan EOS yielded a bulk modulus of K ₀=93(4)?GPa with K′=5.8(1.8). The evolution of the crystal structure of fluorapatite was analysed using data collected at room pressure, at 3.04 and 4.72?GPa. The bulk modulus of phosphate tetrahedron is about three times greater than the bulk modulus of calcium polyhedra. The values were 270(10), 100(4) and 86(3) GPa for P, Ca1 (nine-coordinated) and Ca2 (seven-coordinated) respectively. While the calcium polyhedra became more regular with pressure, the distortion of the phosphate tetrahedron remained unchanged. The size of the channel extending along the [001] direction represented the most compressible direction. The Ca2–Ca2 distance decreased from 3.982 to 3.897?Å on compression from 0.0001 to 4.72?GPa. The anisotropic compressional pattern may be understood in terms of the greater compressibility of the channel size over the polyhedral units. The reduction of the channel volume was measured by the evolution of the trigonal prism, having the Ca2–Ca2–Ca2 triangle as its base and the c lattice parameter as its height. This prism volume changed from 47.3?ų at room pressure to 44.78?ų at 4.72?GPa. Its relatively high bulk moduli, 86(3) GPa, indicated that the channel did not collapse with pressure and the apatite structure could remain stable at very high pressure.     

日本马氏贝珍珠化学组成的同步辐射X射线荧光光谱分析

采用同步辐射微区X射线荧光光谱技术,探测古铜色日本马氏贝珍珠剖面(珠核与珍珠层)的元素分布。测试结果表明,样品中主要含有Ca、Sr、Ba等3种碱土金属元素,Mn、Fe、Cu、Zn等4种3d过渡金属元素以及稀土元素,各元素在珍珠中表现出一定的空间分布规律:珠核表面稀土元素浓度最高,珍珠层和珠核的稀土元素浓度大致相当;Mn和Fe元素浓度自珠核浅表层向珠核表面有降低趋势;珍珠层内Mn和Fe元素浓度都出现大幅降低现象,且是在相同的圈层降低,显示其具有一定正相关性,推测二者大幅降低的圈层是平行层和棱柱层的分界,且Mn元素主要赋存于棱柱层;Ca与Sr、Ba元素的分布具有负相关性。     

Arsenic accumulation in the roots of Helianthus annuus and Zea mays by irrigation with arsenic-rich groundwater: Insights from synchrotron X-ray fluorescence imaging

The aim of this study was to investigate the accumulation of arsenic (As) in and on roots of Zea mays (maize) and Helianthus annuus (sunflower) by means of synchrotron-based micro-focused X-ray fluorescence imaging (μ-XRF). Plant and soil samples were collected from two field sites in the Hetao Plain (Inner Mongolia, China) which have been regularly irrigated with As-rich groundwater. Detailed μ-XRF element distribution maps were generated at the Fluo-beamline of the Anka synchrotron facility (Karlsruhe Institute of Technology) to assess the spatial distribution of As in thin sections of plant roots and soil particles. The results showed that average As concentrations in the roots (14.5–27.4 mg kg⁻¹) covered a similar range as in the surrounding soil, but local maximum root As concentrations reached up to 424 mg kg⁻¹ (H. annuus) and 1280 mg kg⁻¹ (Z. mays), respectively. Importantly, the results revealed that As had mainly accumulated at the outer rhizodermis along with iron (Fe). We therefore conclude that thin crusts of Fe-(hydr)oxides cover the roots and act as an effective barrier to As, similar to the formation of Fe plaque in rice roots. In contrast to permanently flooded rice paddy fields, regular flood irrigation results in variable redox conditions within the silty and loamy soils at our study site and fosters the formation of Fe-(hydr)oxide plaque on the root surfaces.     

Deformation twinning and residual stress in calcite studied with synchrotron polychromatic X-ray microdiffraction

Microstructures of deformed calcite in marble from the Bergell Alps are studied by using a microfocused polychromatic synchrotron X-ray beam. The high spatial resolution, together with orientation and strain resolutions, reveals twin plane orientation, multiple twin lamellae, and strain distributions associated with the twins. Single and multiple mechanical twins on e = { 01[`1] 8 } e = \left\{ {01\overline{1} 8} \right\} systems are confirmed. Residual stresses are derived from the strain tensor that is derived from Laue diffraction patterns. Average lattice strains from several hundred to over one thousand microstrains are detected in a deformed marble from the Bergell Alps. Such strains suggest 60–120 MPa residual stresses. A detailed study of strain components shows that shear stresses on twin planes are completely released.     

The dehydration process in the zeolite laumontite: a real-time synchrotron X-ray powder diffraction study

The dehydration process of the natural zeolite laumontite Ca₄Si₁₆Al₈O₄₈ · 18 H₂O has been studied in situ by means of powder diffraction and X-ray synchrotron radiation. Powder diffraction profiles suitable for Rietveld refinements were accumulated in time intervals of 5 minutes using a position sensitive detector (CPS-120 by INEL), while the temperature increased in steps of about 5 K. The synchronization of accumulation time and temperature plateau allowed collection of 62 temperature-resolved powder patterns in the range 310–584 K, whose analysis produced a dynamic picture of the laumontite structure response to dehydration. The first zeolitic water molecules diffusing out of the channels are those not bonded to the Ca cations and located in the W(1) site, whose occupancy drops smoothly to 10% during heating to 349 K, while the sample in the capillary is still submerged in water. The remaining W(1) and 60% of W(5) water molecules are expelled rather sharply at about 370 K. At this temperature all remaining water submerging the powder crystallites is lost, the structure contains about 13 water molecules/cell, and the crystal structure is that of leonhardite. On continued heating 80% of the water molecules from the W(2) site are lost between 420 and 480 K, while a small amount of the diffusing water is reinserted in the W(5) site. The occupancy factor of the W(8) site decreases starting at 480 K, and reaches a maximum loss of 20% at 584 K. The combined occupancy of the Ca-coordinated W (2) and W (8) water sites never falls much below two, so that the Ca cations in the channels, which are bonded to four framework oxygen atoms, are nearly six-coordinated in the explored temperature range. The water loss is accompanied by large changes in the unit cell dimensions. Except at 367 K, where the excess surrounding water is leaving, all changes in cell dimensions are gradual. The loss of the hydrogen bonded W(1) and W(5) water molecules is related to most of the unit cell volume reduction below 370 K, as shown by the contraction of the a-, b- and c-axes and the increase in the monoclinic angle. Loss of the Ca-coordinated W(2) and W(8) water molecules has a small effect on the unit cell volume as the continued contraction of the a- and c-axes is counter-balanced by a large expansion in the b-axis and a decrease in the monoclinic β angle.     

Growth of magnesio-aluminate spinel in thin-film geometry: in situ monitoring using synchrotron X-ray diffraction and thermodynamic model

Polycrystalline spinel layers were grown experimentally at the contacts between single-crystal corundum substrates and initially amorphous, then polycrystalline MgO thin films. The growth behavior of the spinel layers was monitored in situ using synchrotron X-ray diffraction. The change in the integrated intensity of the 111 spinel Bragg peak was correlated with the thickness of the layer as determined from ex situ TEM characterization of the run products. At \(900\,^{\circ }\hbox {C},\) a transition from linear growth, corresponding to interface reaction control, to parabolic growth, corresponding to diffusion control, occurred at a layer thickness of less than 10 nm. At 1,000 \(^{\circ }\hbox {C},\) growth was largely linear up to a layer thickness in excess of 300 nm. A thermodynamic model was applied to extract the kinetic parameters characterizing interface motion and long-range diffusion from this growth behavior.     

Phase contrast synchrotron X-ray microtomography of Ediacaran (Doushantuo) metazoan microfossils: Phylogenetic diversity and evolutionary implications

Microfossils from the Ediacaran Weng’an Phosphate Member of the Doushantuo Formation (Guizhou Province, southern China) have received widespread attention. The Doushantuo, which overlies the glacial deposits of the Nantuo Formation, was deposited following the Marinoan glaciation, the last extensive glaciation of Snowball Earth. Radiometric age dating indicates that the Doushantuo is older than 580 my, and hence that these microfossils are older than the Ediacara Biota. However, the diversity represented by these fossils has yet to be fully documented. A recent technological approach that has increasingly been used to image fossils, propagation phase contrast synchrotron X-ray microtomography, has allowed non-destructive study of both exterior and interior features of a variety of Doushantuo microfossils from the gray facies of the Weng’an Phosphate Member, cropping out along the axis of the Mt. Beidou anticline. Studies of Doushantuo embryos demonstrate the existence of a large suite of modern embryonic features, including macromeres and micromeres, cell lineage, polar lobes, compacted epithelia, equal and unequal cleavage, blastulation and gastrulation, and chorionic protection. Because embryos such as those here studied provide only a limited amount of phylogenetic information, and because adult metazoans of the types that produced these embryos have yet to be discovered in Doushantuo-age rocks, these fossilized embryonic forms can at present be assigned only to the various superclades represented amongst living Metazoa. The diversity of the embryos here studied suggests that the metazoan fauna of the Doushantuo may well have included animals of poriferan, cnidarian, and both protostomial (representatives possibly of basal protostome lineages) and deuterostomial affinity. If this interpretation is correct, it would then follow that the last common ancestor of the bilaterian metazoan lineage, as well as the last common ancestor of sponges, cnidarians and bilaterians, pre-dated deposition of the Doushantuo strata.     

Structural states of natural potassium feldspar: An infrared spectroscopic study

Natural samples of K-feldspar representing various states of Al, Si order were characterised using X-ray methods, transmission electron microscopy, and Fourier transform infrared spectroscopy. Line profiles of infrared absorption bands were observed to show strong correlation with the degree of Al, Si order present. In particular, the absorption frequencies of the 540 cm^?1 and 640 cm^?1 bands were seen to vary by ca. 10 cm^?1 between sanidine and microcline, with modulated samples respresenting intermediate behaviour. Linewidths of these modes also decrease by ca. 50% in this series. The experimental results are discussed within the framework of Hard Mode Infrared Spectroscopy (HMIS), and it is shown that the absorption frequencies vary with the short range order parameter τ = (4t₁-1)² and the symmetry breaking order parameter describing Al, Si order, Q _od=(t₁ 0?t₁ m)/Q _od=(t₁ 0+t₁ m), where t₁ is the average Al occupancy on the T₁ sites and t₁ o and t₁ m are the individual site occupancies of the T₁ o and T₁ m sites, respectively. The structural state of orthoclase is characterised by strain-induced modulations with large spatial variations of the modulation wavelength. No such modulations were observed in the degree of local Al, Si order. Sanidine shows mode hardening in excess of the extrapolated effect of symmetry breaking Al, Si order, which is presumably related to nonsymmetry breaking ordering between T₁ and T₂ sites and/or as yet unobserved short range order of the symmetry breaking ordering scheme. The possibility of an additional phase transition in K-feldspar at temperatures above 1300 K is discussed.     

Electron density distribution and bond critical point properties for forsterite, Mg2 SiO4, determined with synchrotron single crystal X-ray diffraction data

A generalized X-ray scattering factor model experimental electron density distribution has been generated for the orthosilicate forsterite, using an essentially extinction and absorption free set of single crystal diffraction data recorded with intense, high energy synchrotron X-ray radiation (E=100.6 keV). A refinement of the model converged with an R(F)=0.0061. An evaluation of the bond critical point, bcp, properties of the distribution at the (3, –1) stationary points for the SiO and MgO bonded interactions, yielded values that agree typically within ~5%, on average, with theoretical values generated with quantum chemical computational strategies, using relatively robust basis sets. On the basis of this result, the modeling of the experimental distribution is considered to be adequate. As the bcp properties increase in magnitude, the MgO and SiO bonds decrease in length as calculated for a number of rock forming silicates. As asserted by Coppens (X-ray charge densities and chemical bonding. Oxford University Press, Oxford, 1997), large negative ²(r_c) values, characteristic of shared interactions involving first row atoms, may not be characteristic of closed shell covalent bonded interactions involving second row Si, P and S atoms bonded to O. This study adds new evidence to the overall relatively good agreement between theoretical bcp properties generated with computational quantum strategies, on the one hand, and experimental properties generated with single crystal high energy synchrotron diffraction data on the other. The similarity of results not only provides a basis for using computational strategies for studying and modeling structures, defects and the reactivity of representative structures, but it also provides a basis for improving our understanding of the crystal chemistry of earth materials and the character of the SiO bonded interaction.     

Analysis of individual fluid inclusions using synchrotron X-ray fluorescence microprobe: progress toward calibration for trace elements

A critical problem for conducting quantitative analysis of individual fluid inclusions using Synchrotron X-Ray Fluorescence (SXRF) technique relates to the standardization and the calibration of the X-ray spectra. In this study, different approaches have been tested for calibration purposes: (a) the use of chlorine when Cl content can be estimated either from melting point depressions of undersaturated fluid inclusions or from saturation limits for halite-bearing fluid inclusions, (b) the use of calcium from synthetic fluid inclusions of known CaCl₂ content as an external standard. SXRF analysis was performed on individual fluid inclusions from the Chivor and Guali emerald deposits, Columbia. These well-known samples contain a single fluid inclusion population for which detailed crush-leach analyses are available, thus providing a relevant compositional reference frame. Concentration estimates were also compared to Particle Induced X-ray Emission (PIXE) analysis carried out independently on the same fluid inclusions.Results of the calibration tests indicate that major (Cl, K, Ca, Fe, Mn) and trace element (Cu, Zn, As, Br, Rb, Sr, Ba, Pb) concentration estimates can be performed without precise knowledge of the analytical volume and the inclusion’s 3D geometry. Although the standard deviation of the SXRF results can be relatively high depending on the calibration mode used, mean concentration estimates for most elements are in good agreement with PIXE and crush-leach analysis. Elemental distributions within single fluid inclusions were also established. Associated correlation diagrams argue for the homogeneous distribution of most elements in the fluid inclusion. In contrast, Br shows a bimodal distribution interpreted to reflect a significant enrichment of the vapor portion of the inclusion fluid.