The long-term fate of Cu, Zn, and Pb adsorption complexes at the calcite surface: An X-ray absorption spectroscopy study

In this study, the speciation of Zn²⁺, Pb²⁺, and Cu²⁺ ions sorbed at the calcite surface was monitored during a 2.5-year reaction period, using extended X-ray absorption spectroscopy to characterize metal speciation on the molecular scale. Experiments were performed using pre-equilibrated calcite-water suspensions of pH 8.3, at metal concentrations below the solubility of metal hydroxide and carbonate precipitates, and at constant metal surface loadings. The EXAFS results indicate that all three metals remained coordinated at the calcite surface as inner-sphere adsorption complexes during the 2.5-year ageing period, with no evidence to suggest slow formation of dilute metal-calcite solid solutions under the reaction conditions employed. All three divalent metals were found to form non-octahedral complexes upon coordination to the calcite surface, with Zn²⁺ adsorbing as a tetrahedral complex, Cu²⁺ as a Jahn-Teller distorted octahedral complex, and Pb²⁺ coordinating as a trigonal- or square-pyramidal surface complex. The non-octahedral configurations of these surface complexes may have hindered metal transfer from the calcite surface into the bulk, where Ca²⁺ is in octahedral coordination with respect to first-shell O. The use of pre-equilibrated calcite suspensions, with no net calcite dissolution or precipitation, likely prevented metal incorporation into the lattice as a result of surface recrystallization. The results from this study imply that ageing alone does not increase the stability of Zn²⁺, Pb²⁺, and Cu²⁺ partitioning to calcite if equilibrium with the solution is maintained during reaction; under these conditions, these metals are likely to remain available for exchange even after extended sorption times.     

X-ray absorption spectroscopy study of Cu and Zn adsorption complexes at the calcite surface: Implications for site-specific metal incorporation preferences during calcite crystal growth

We report results from in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Cu(II) and Zn(II) complexes forming at the calcite surface following adsorption from preequilibrated calcite-saturated solutions. Both Cu(II) and Zn(II) coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes. The Zn adsorption complexes are in tetrahedral coordination with first-shell O neighbors with R_Zn-O = 1.95 Å, and the Cu complexes are Jahn-Teller distorted, with equatorial R_Cu-O = 1.95 Å. Results from EXAFS data of dilute Cu- and Zn-calcite solid solutions confirm substitution of these metals in the Ca site of the calcite structure as octahedral complexes during coprecipitation. X-ray fluorescence microanalyses of calcite (101?4) hillocks grown in coprecipitation experiments show that divalent Cu and Zn, which have ionic radii smaller than Ca, are preferentially incorporated into the parallel arrays of <4?41>₊ steps that define one pair of symmetrically equivalent vicinal faces on polygonized growth spirals. In contrast, other divalent metals with sixfold ionic radii smaller than Ca (Co, Cd, Mn, Mg) have been shown to be preferentially incorporated into <4?41>₋ growth steps, which define the second pair of vicinal faces on the growth spirals, but which are symmetrically nonequivalent to the steps on the first pair. The distortion from octahedral symmetry observed for the Cu and Zn adsorption complexes likely plays a key role in the observed preference of Cu and Zn for incorporation into the <4?41>₊ steps.     

X射线吸收谱在宝石学研究中的应用

许多宝玉石的颜色成因都与致色元素的电子结构和局域环境相关.X射线吸收谱(XAS)是一种探测物质结构的有力工具,能够识别吸收原子的局域结构和化学环境,如价态、配位数、键长等.本文以红宝石、金绿宝石、翡翠和象牙等为例,简要介绍同步辐射XAS技术在宝石研究中的应用和优势.     

High-pressure behaviour of germanate olivines studied by X-ray diffraction and X-ray absorption spectroscopy

Germanate olivines Mg₂GeO₄, Ca₂GeO₄ and CaMgGeO₄ have been studied by high-pressure X-ray Diffraction and high-pressure X-ray Absorption Spectroscopy. The three compounds were compressed, in the 0–30 GPa pressure range, at room temperature in a diamond-anvil cell, silicon oil being used as the pressure transmitting medium. Values of K₀ are 166 ± 15, 117 ± 15 and 152 ± 14 GPa for Mg₂GeO₄, Ca₂GeO₄ and CaMgGeO₄ respectively. These olivines all exhibit compression anisotropy, the a axis being the least compressible. Crystal to crystal phase transitions have been observed in Mg₂GeO₄ and Ca₂GeO₄ above 12 GPa and 6 Gpa respectively. The nature of these structural changes remains unclear yet. The onset of amorphization has been observed in Mg₂GeO₄ and Ca₂GeO₄ at pressures above about 22 and 11 GPa respectively. These phase transitions and amorphization processes do not involve any detectable increase in the coordination number of germanium atoms. At higher pressure (P >23 GPa), we report the onset of a transition from a phase with fourfold coordinated germanium to a phase with higher germanium coordination number in CaMgGeO₄.     

Polarized X-ray absorption spectroscopy of metal ions in minerals

We have made use of the nearly complete linear polarization of synchrotron radiation to study the polarization dependence of X-ray absorption near-edge structure (XANES) and extended fine structure (EXAFS) in oriented single crystals of gillespite (BaFe²⁺ Si₄O₁₀; Fe^{2 +} in square-planar coordination, point symmetry C ₄), anatase (TiO₂; Ti⁴⁺ in octahedral coordination, point symmetry D _2d), and epidote (Ca₂(Al, Fe³⁺)₃SiO₄)₃(OH); Fe³⁺ in distorted octahedral coordination, point symmetry (C _s). For gillespite, the Fe K-XANES spectrum varies strongly with E-vector orientation of the incident X-ray beam. When the E-vector lies in the plane of the FeO₄ group (i.e., perpendicular to the c-axis), multiple-scattering features at 7127 and 7131 eV intensify, whereas when the E-vector is perpendicular to the plane of the FeO₄ group (i.e., parallel to the c-axis), a strongly-polarized 1s to 4p bound state transition occurs at 7116 eV and a localized continuum resonance occurs at 7122 eV. The Fe-K-EXAFS spectrum of gillespite is also highly polarization dependent. When the E-vector is perpendicular to c, all four nearest-neighbor oxygens around Fe²⁺ contribute to the EXAFS signal; when E is parallel to c, the EXAFS signal from nearest-neighbors is reduced by at least 86%. The unpolarized Ti K-XANES spectrum of anatase has three relatively strong pre-edge features at 4967.1, 4969.9, and 4972.7 eV which have resisted definitive interpretation in past studies. The lowest energy feature has a strong xy polarization dependence, suggesting a large amount of 4p _x,y character, and it is also very sharp, indicating a well-defined transition energy. Both of these observations are consistent with an excitonic state with a binding energy of 2.8 eV. The two higher energy features, which are characteristic of octahedrally-coordinated Ti⁴⁺, show little polarization dependence and are probably due to 1s to 3d bound-state transitions, with a small degree of np character in the final state wavefunction. Interpretation of the polarization dependence of Fe K-XANES spectra for epidote is not as straightforward due to the lower space group symmetry (P2₁/m) relative to gillespite (P4/ncc) and anatase (I4₁/amd) and the lower point group symmetry (C _s) of the M(3) site which contains most of the Fe³⁺ in the epidote structure. However, the presence of a shoulder at 7121 eV in the E parallel to b spectrum and its absence in the E normal to bc spectrum are consistent with it being a 1s to 4p _z bound-state transition. Strong, weakly x, y polarized features near 7126 eV in both spectra are most likely due to localized continuum transitions. Also, the 1s to 3d pre-edge intensity varies in intensity with E-vector orientation which is consistent with displacement of Fe³⁺ from the center of the M(3) octahedral site. Analysis of EXAFS spectra of epidote in these two polarizations yields bond distances which are within 0.04 Å of previous single-crystal X-ray diffraction analysis. This study demonstrates the utility of polarized X-ray absorption spectroscopy in quantifying the energies and orbital compositions of final state wavefunctions associated with various X-ray induced transitions in transition-metal containing minerals. It also shows that reasonably accurate M-O distances can be obtained for individual bonds oriented in crystallographically non-equivalent directions.     

X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations

Current research on bioremediation of uranium-contaminated groundwater focuses on supplying indigenous metal-reducing bacteria with the appropriate metabolic requirements to induce microbiological reduction of soluble uranium(VI) to poorly soluble uranium(IV). Recent studies of uranium(VI) bioreduction in the presence of environmentally relevant levels of calcium revealed limited and slowed uranium(VI) reduction and the formation of a Ca-UO₂-CO₃ complex. However, the stoichiometry of the complex is poorly defined and may be complicated by the presence of a Na-UO₂-CO₃ complex. Such a complex might exist even at high calcium concentrations, as some UO₂-CO₃ complexes will still be present. The number of calcium and/or sodium atoms coordinated to a uranyl carbonate complex will determine the net charge of the complex. Such a change in aqueous speciation of uranium(VI) in calcareous groundwater may affect the fate and transport properties of uranium. In this paper, we present the results from X-ray absorption fine structure (XAFS) measurements of a series of solutions containing 50 μM uranium(VI) and 30 mM sodium bicarbonate, with various calcium concentrations of 0-5 mM. Use of the data series reduces the uncertainty in the number of calcium atoms bound to the UO₂-CO₃ complex to approximately 0.6 and enables spectroscopic identification of the Na-UO₂-CO₃ complex. At nearly neutral pH values, the numbers of sodium and calcium atoms bound to the uranyl triscarbonate species are found to depend on the calcium concentration, as predicted by speciation calculations.     

Coprecipitation of radionuclides with calcite: estimation of partition coefficients based on a review of laboratory investigations and geochemical data

Coprecipitation of radionuclides with secondary solids is currently neglected in safety assessments for radioactive waste repositories, although this process is thought to be important in limiting radionuclide solution concentrations. This paper provides a systematic review of laboratory data on metal coprecipitation with calcite, presented in the form of phenomenological partition coefficients. The aim of this investigation is to provide a consistent set of parameter values for the quantitative modelling of radionuclide coprecipitation with calcite, which will be the dominant alteration product in cementitious repositories accessed by carbonate-rich groundwater.From the data reviewed, empirical correlations have been derived that relate experimentally determined partition coefficients (λ_Me) to measurable chemical properties of the coprecipitated metals (ionic radii and sorption parameters of the incorporated trace metals, solubility products of the pure metal carbonates). These correlations have then been used to predict the partition coefficients of radionuclides for which no laboratory data exist.Such predictions indicate that the actinides will partition strongly into calcite under reducing conditions (λ_Me ∼200–1000 for trivalent, λ_Me ∼20–200 for tetravalent actinides). Nickel(II) incorporation will be moderate (λ_Me ∼1–10), while incorporation of ions like U(VI), Cs(I), Sr(II) and Ra(II) in calcite will be weak (λ_Me<1).In spite of substantial uncertainties, the estimated partition coefficients are sufficiently accurate to allow a semi-quantitative evaluation of the effect of radionuclide coprecipitation with calcite in limiting radionuclide solution concentrations in well characterised repository environments.     

Iron local structure in tektites and impact glasses by extended X-ray absorption fine structure and high-resolution X-ray absorption near-edge structure spectroscopy

The local structure of iron in three tektites has been studied by means of Fe K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution X-ray absorption near-edge structure (XANES) spectroscopy in order to provide quantitative data on <Fe-O> distance and Fe coordination number. The samples studied are a moldavite and two australasian tektites. Fe model compounds with known Fe oxidation state and coordination number were used as standards in order to extract structural information from the XANES pre-edge peak. EXAFS-derived grand mean <Fe-O> distances and Fe coordination numbers for the three tektite samples are constant within the estimated error (<Fe-O > =2.00 Å ± 0.02 Å, CN = 4.0 ± 0.4). In contrast to other data from the literature on Fe-bearing silicate glasses, the tektites spectra could not be fitted with a single Fe-O distance, but rather were fit with two independent distances (2 × 1.92 Å and 2 × 2.08 Å). High-resolution XANES spectra of the three tektites display a pre-edge peak whose intensity is intermediate between those of staurolite and grandidierite, thus suggesting a mean coordination number intermediate between 4 and 5. Combining the EXAFS and XANES data for Fe, we infer the mean coordination number to be close to 4.5.Comparison of the tektites XANES spectra with those of a suite of different impact glasses clearly shows that tektites display a relatively narrow range of Fe oxidation state and coordination numbers, whereas impact glasses data span a much wider range of Fe oxidation states (from divalent to trivalent) and coordination numbers (from tetra-coordinated to esa-coordinated). These data suggest that the tektite production process is very similar for all the known strewn fields, whereas impact glasses can experience a wide variety of different temperature-pressure-oxygen fugacity conditions, leading to different Fe local structure in the resulting glasses. These data could be of aid in discriminating between tektite-like impact glasses and impact glasses sensu strictu.     

Sulfur K- and L-edge X-ray absorption spectroscopy of sphalerite,chalcopyrite and stannite

Sulfur K-edge x-ray absorption spectra (XANES and EXAFS) and L-edge XANES of sphalerite (ZnS), chalcopyrite (CuFeS₂) and stannite (Cu₂FeSnS₄) have been recorded using synchrotron radiation. The K- and L-edge XANES features are interpreted using a qualitative MO/energy band structure model. The densities of unoccupied states at the conduction bands of sphalerite, chalcopyrite and stannite are determined using S K- and L-edge XANES features (up to 15 eV above the edge), combined with published metal K-edge XANES. The SK- and L-edge XANES also indicate that, for sphalerite, the Fe²⁺ 3d band at the fundamental gap has little or no bonding hybridization with S 3p and S 3s orbitals; for chalcopyrite, the Cu⁺ 3d and Fe³⁺ 3d bands have strong mixing with S 3p and S 3s states, while for stannite the Cu⁺ 3d band strongly hybridizes with S 3p and S 3s orbitals, but the Fe²⁺ 3d band does not. The post-edge XANES features (15–50 eV above the edge) of sphalerite, chalcopyrite and stannite are similar. These features are related to the tetrahedral coordination of sulfur in all these structures, and interpreted by a multiple scattering model. The resonance energies from both the K-edge and L-edge XANES for these minerals are well correlated with reciprocal interatomic distances and lattice spaces. Sulfur K-edge EXAFS analyses using Fourier transform and curve fitting procedures are presented. Comparison of the structural parameters from EXAFS with x-ray structure data shows that the first shell bond distances (BD) from EXAFS are usually accurate to ±0.02 Å, and that coordination numbers (CN) are generally accurate to ±20 percent. For sphalerite, EXAFS analysis yields the structure parameters for the first three neighbour shells around a sulfur atom; the BD and CN even for the third shell are in close agreement with the x-ray structure, and the Debye-Waller term decreases from the first shell to the third shell. It is shown that sphalerite (ZnS) is a good model compound for EXAFS analysis of sulfur in chalcogenide glasses and metalloproteins.     

An X-ray absorption spectroscopy study of Cd binding onto bacterial consortia

In this study, we use extended X-ray absorption fine structure (EXAFS) spectroscopy measurements to examine the atomic environment of Cd bound onto two experimental bacterial consortia: one grown from river water, and one grown from a manufacturing gas plant site. The experiments were conducted as a function of pH and demonstrate that the complex mixtures of bacteria, containing both Gram-positive and Gram-negative species, yield relatively simple EXAFS spectra, a result which indicates that only a limited number of functional group types contribute to Cd binding for each bacterial consortium. The EXAFS spectra indicate that the average Cd binding environment in the river water consortium varies significantly with pH, but the manufacturing gas plant consortium exhibits a Cd binding environment that remains relatively constant over the pH range examined. The EXAFS data for the river water consortium were modeled using carboxyl, phosphoryl and sulfhydryl sites. However, only carboxyl and phosphoryl sites were required to model the manufacturing gas plant consortium data under similar experimental conditions. This is the first EXAFS study to identify and quantify the relative importance of metal binding sites in bacterial consortia. Although our results indicate differences in the binding environments of the two consortia, the data suggest that there are broad similarities in the binding environments present on a wide range of bacterial cell walls.     

Crystal chemistry of iron in natural grandidierites: an X-ray absorption fine-structure spectroscopy study

 Fe–K edge XAFS spectra (pre-edge, XANES and EXAFS) were collected for eight grandidierites from Madagascar and Zimbabwe, as well as for Fe(II) and Fe(III) model compounds (staurolite, siderite, enstatite, berlinite, yoderite, acmite, and andradite). The pre-edge spectra for these samples are consistent with dominantly 5-coordinated ferrous iron. The analysis of the XANES and EXAFS spectra confirms that Fe(II) substitutes for Mg(II) in grandidierite, with a slight expansion of the local structure around Mg by ∼2%. In addition, ferric iron was also detected in some samples [5–10 mol% of the total Fe or 500–1100 ppm Fe(III)]. Based on theoretical calculations of the EXAFS region, Fe(III) appears to be located in the 5-coordinated sites of Mg(II) or in the most distorted 6-coordinated sites of Al (depending on the sample studied). Special attention is therefore required when using grandidierite as a model for ferrous iron in C_3v geometry, because of the possible presence of an extra contribution related to Fe(III). This additional contribution enhances significantly the Fe–K pre-edge integrated area [+40% for 1000 ppm Fe(III)]. Therefore, only a few grandidierite samples can be used as a robust structural model for the study of the Fe(II) coordination in glasses and melts. Received: 26 June 2000 / Accepted: 19 February 2001     

Arsenic and iron speciation in uranium mine tailings using X-ray absorption spectroscopy

In northern Saskatchewan, Canada, high-grade U ores and the resulting tailings can contain high levels of As. An environmental concern in the U mining industry is the long-term stability of As within tailings management facilities (TMFs) and its potential transfer to the surrounding groundwater. To mitigate this problem, U mill effluents are neutralized with lime to reduce the aqueous concentration of As. This results in the formation of predominantly Fe³⁺–As⁵⁺ secondary mineral phases, which act as solubility controls on the As in the tailings discharged to the TMF. Because the speciation of As in natural systems is critical for determining its long-term environmental fate, characterization of As-bearing mineral phases and complexes within the deposited tailings is required to evaluate its potential transformation, solubility, and long-term stability within the tailings mass. In this study, synchrotron-based bulk X-ray absorption spectroscopy (XAS) was used to study the speciation of As and Fe in mine tailings samples obtained from the Deilmann TMF at Key Lake, Saskatchewan. Comparisons of K-edge X-ray absorption spectra of tailings samples and reference compounds indicate the dominant oxidation states of As and Fe in the mine tailings samples are +5 and +3, respectively, largely reflecting their generation in a highly oxic mill process, deposition in an oxidized environment, and complexation within stable oxic phases. Linear combination fit analyses of the K-edges for the Fe X-ray absorption near edge spectra (XANES) to reference compounds suggest Fe is predominantly present as ferrihydrite with some amount of the primary minerals pyrite (8–15% in some samples) and chalcopyrite (5–15% in some samples). Extended X-ray absorption fine structure (EXAFS) analysis of As K-edge spectra indicates that As⁵⁺ (arsenate) present in tailings samples is adsorbed to the ferrihydrite though an inner-sphere bidentate linkage.     

同步辐射软X射线吸收谱与发射谱测定天然针铁矿能带结构

天然半导体矿物由于成分、缺陷复杂,传统测试方法如紫外可见漫反射等难以准确测定其禁带宽度.本文以针铁矿为例,通过第一性原理计算得到纯针铁矿及掺Al针铁矿的电子结构.计算结果显示,纯针铁矿导带底与价带顶均由Fe3d与O2p轨道组成,而当含杂质Al时,Al2p与O2p发生杂化参与了价带组成.在此基础上,利用同步辐射X射线氧的K边吸收谱与发射谱对纯针铁矿及天然针铁矿的能带结构进行了测定.结果表明,天然含Al的针铁矿禁带宽度为2.30eV,小于纯针铁矿(2.57eV).本研究提供了一种测定天然氧化物矿物禁带宽度的新方法,为深入研究天然半导体可见光催化活性产生机制提供了理论依据.     

Molecular environment of iodine in naturally iodinated humic substances: Insight from X-ray absorption spectroscopy

The molecular environment of iodine in reference inorganic and organic compounds, and in dry humic and fulvic acids (HAs and FAs) extracted from subsurface and deep aquifers was probed by iodine L₃-edge X-ray absorption spectroscopy. The X-ray absorption near-edge structure (XANES) of iodine spectra from HAs and FAs resembled those of organic references and displayed structural features consistent with iodine forming covalent bonds with organic molecules. Simulation of XANES spectra by linear combination of reference spectra suggested the predominance of iodine forming covalent bonds to aromatic rings (aromatic-bound iodine). Comparison of extended X-ray absorption fine structure (EXAFS) spectra of reference and samples further showed that iodine was surrounded by carbon shells at distances comparables to those for references containing aromatic-bound iodine. Quantitative analysis of EXAFS spectra indicated that iodine was bound to about one carbon at a distance d (I-C) of 2.01(4)-2.04(9) Å, which was comparable to the distances observed for aromatic-bound iodine in references (1.99(1)-2.07(6) Å), and significantly shorter than that observed for aliphatic-bound iodine (2.15(2)-2.16(2) Å). These results are in agreement with previous conclusions from X-ray photoelectron spectroscopy and from electrospray ionization mass spectrometry. These results collectively suggest that the aromatic-bound iodine is stable in the various aquifers of this study.     

Surface modification study of borate materials from B K-edge X-ray absorption spectroscopy

The B K-edge X-ray absorption near-edge structure (XANES) spectra of two borates with tetrahedrally-coordinated B [^[4]B; natural danburite (CaB₂Si₂O₈) and synthetic boron phosphate (BPO₄)] have been recorded in total electron yield (TEY) and fluorescence yield (FY) modes to investigate the surface and bulk structure of these materials. The TEY XANES measurement shows that danburite is susceptible to surface damage involving conversion of ^[4]B sites to ^[3]B sites by reaction with moisture and/or mechanical abrasion (grinding, polishing, etc.). The bulk of the mineral is essentially unaffected. Commercial boron phosphate powder exhibits more extensive surface and bulk damage, which increases with air exposure but is recovered on heating at 650°C. In contrast to ELNES, the XANES technique is not affected by beam damage and when collected in the FY mode is capable of yielding meaningful information on the coordination and intermediate-range structure of B in borate and borosilicate materials. Received: 7 April 1997 / Accepted: 30 July 1997     

Polycrystalline calcite to aragonite transformation kinetics: experiments in synthetic systems

The kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates, with and without additional minerals present. The reaction progresses as a function of time were measured at four temperature/pressure conditions: (1) 550 °C/1.86 GPa; (2) 600 °C/2.11 GPa; (3) 650 °C/2.11 GPa, and (4) 700 °C/2.29 GPa. Experiments reveal that Mg-calcite and Fe-calcite transforms to aragonite at considerably slower rates than pure calcite, and that Sr-bearing calcite and calcite + quartz aggregates transform at significantly higher rates than pure calcite. The reaction progresses vs. time data for pure calcite were fitted to Cahns grain-boundary nucleation and interface-controlled growth model. Evidence for interface-controlled growth is provided by petrographic observations of grain boundaries. The activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble. The discrepancy in rates and activation energy may be attributed to the nature of grain boundaries, to deformational strain or the presence of impurities in the studied samples, and likely to uncertainties in experimental conditions. The results of this study imply that the variation of local petrologic conditions, in addition to temperature, pressure and grain size, may play an important role in determining the rates for the calcite to aragonite transformation in nature.Editorial responsibility: W. Schreyer     

Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy

X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe²⁺ and Fe³⁺ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe²⁺-O bond distance, closely matching octahedral Fe²⁺-O bonds obtained from solid oxide model compounds. Solution Fe²⁺-O bond distances decrease with chloride/iron ratio, pH, and total FeCl₂ concentration. A slight intensification of the $\text{1s → 3d}$ transition with increasing FeCl₂ concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe³⁺ solutions show a pronounced increase in the $\text{1s → 3d}$ transition intensities between 1.0 M FeCl₃/7.8 M Cl^? to 1.0 M FeCl₃/ 15 M Cl^?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe³⁺-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H₂O)₆³⁺ or trans-Fe(H₂O)₄Cl₂ or both; Fe³⁺-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe³⁺-Cl bond distances of 2.25 Å].     

Basal slip and texture development in calcite: new results from torsion experiments

The deformation behavior of calcite has been of longstanding interest. Through experiments on single crystals, deformation mechanisms were established such as mechanical twinning on in the positive sense and slip on and both in the negative sense. More recently it was observed that at higher temperatures slip in both senses becomes active and, based on slip line analysis, it was suggested that slip may occur. So far there had been no direct evidence for basal slip, which is the dominant system in dolomite. With new torsion experiments on calcite single crystals at 900 K and transmission electron microscopy, this study identifies slip unambiguously by direct imaging of dislocations and diffraction contrast analysis. Including this slip system in polycrystal plasticity simulations, enigmatic texture patterns observed in compression and torsion of calcite rocks at high temperature can now be explained, resolving a long-standing puzzle.