Alan J. Anderson

HYDROTHERMAL DIAMOND ANVIL CELL FOR XAFS STUDIES OF FIRST-ROW TRANSITION ELEMENTS IN AQUEOUS SOLUTION UP TO SUPERCRITICAL CONDITIONS

Abstract A hydrothermal diamond anvil cell (HDAC) has been modified by drilling holes with a laser to within 150 μm of the anvil face to minimize the loss of X-rays due to absorption and scatter by diamond. This modification enables acquisition of K-edge X-ray absorption fine structure (XAFS) spectra from first-row transition metal ions in aqueous solutions at temperatures ranging from 25°C to 660°C and pressures up to 800 MPa. These pressure–temperature ( P – T ) conditions are more than sufficient for carrying out experimental measurements that can provide data valuable in the interpretation of fluid inclusions in minerals found in ore-forming hydrothermal systems as well as other important lithospheric processes involving water.

Assessment of the uncertainties and limitations of quantitative elemental analysis of individual fluid inclusions using synchrotron X-ray fluorescence (SXRF)

Abstract Synchrotron X-ray Fluorescence (SXRF) analysis is a nondestructive analytical technique that provides compositional information from single fluid inclusions. A protocol for conducting quantitative analyses of metal concentrations in individual fluid inclusions has been developed. This has led to an understanding of the accuracy, precision, and detection limits of this technique, as well as the optimal shapes, sizes, and geometries required for reliable fluid inclusion analysis. Aqueous fluid inclusions containing known concentrations of SrCl 2 were synthesized for the development and the standardization of this technique. Strontium chloride was selected because it is highly soluble, its freezing-point depression is well known (allowing us to confirm the inclusion composition using microthermometric analyses), and the energetic Sr X-rays are only mildly attenuated by quartz. To confirm the composition of the synthetic standards, solutions were measured before and after each hydrothermal run using Atomic Absorption Spectroscopy (AAS), and the freezing-point depression for each fluid inclusion was measured. SXRF analyses were performed on beam line X26A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory using an 8 × 12 μm white X-ray beam. The analytical volume was calculated based on known beam dimensions and fluid inclusion geometry determined using a modified spindle stage. Elemental concentrations were determined by ratioing the Sr counts from an inclusion to the counts obtained from capillaries of known diameter containing similar solutions. Numerous inclusions from five samples, each with a different Sr concentration, were analyzed. Within a single population the mean is very close to the known concentration, but the precision is poor, with standard deviations (lσ) from 10–39% of the mean. Errors in determining the inclusion geometry are the main contributor to the poor precision. The poor precision requires that numerous inclusions within one population be analyzed and averaged to accurately estimate the metal concentration for that population. Selection of flat-lying, equant, regularly-shaped inclusions for analysis minimizes errors resulting from inclusion geometry if quantitative results are sought. The detection limit for Sr in synthetic fluid inclusions (typically 4–15 μm thick, and 5–100 pm below the upper polished surface) is approximately 2,000 ppm Sr.

X-ray spectroscopic investigations of fluids in the hydrothermal diamond anvil cell: The hydration structure of aqueous La3+ up to 300 °C and 1600 bars

Abstract The first direct measurements are reported for the structure of the hydrated La3+ ion in an aqueous solution (containing 0.007 m La) over a range of temperatures from 25 to 300 °C and pressures up to 1600 bars. The radial distribution of atoms around the La3+ ion was measured using the X-ray absorption fine structure (XAFS) technique. La L3-edge spectra were collected in the fluorescence mode from nitrate solutions in a modified hydrothermal diamond anvil cell using the PNC-CAT Xray microprobe at the Advanced Photon Source, Argonne National Laboratory. Analysis of the XAFS spectra collected at all temperatures indicates that each La3+ ion has a hydration number of nine and that the solvating waters surround the ion in a tricapped trigonal prismatic arrangement. As temperature is increased from 25 to 300 °C, the bond distance between the equatorial-plane O atoms and the La3+ ion increases from 2.59 ± 0.02 to 2.79 ± 0.04 Å, whereas the bond distance between La3+ and the O atoms at the ends of the prism decrease to 2.48 ± 0.03 Å. This study also demonstrates the unique capability of the modified hydrothermal diamond anvil cell for in situ low energy X-ray spectroscopic analysis of elements in dilute aqueous solutions at elevated temperatures and pressures.

XAFS measurements on zinc chloride aqueous solutions from ambient to supercritical conditions using the diamond anvil cell.

The structure and bonding properties of metal complexes in subcritical and supercritical fluids are still largely unknown. Conventional high pressure and temperature cell designs impose considerable limitations on the pressure, temperature, and concentration of metal salts required for measurements on solutions under supercritical conditions. In this study, we demonstrate the first application of the diamond anvil cell, specially designed for x-ray absorption studies of fnst-row transition metal ions in supercritical fluids. Zn K-edge XAFS spectra were measured from aqueous solutions of 1-2m ZnC12 and up to 6m NaC1, at temperatures ranging from 25-660 °C and pressures up to 800 MPa. Our results indicate that the ZnC142complex is predominant in the lm ZnC12/6m NaC1 solution, while ZnC12(H20)2 is similarly predominant in the 2m ZnC12 solution, at all temperatures and pressures. The Zn-C1 bond length of both types of chlorozinc(II) complexes was found to decrease at a rate of about 0.01 A/100 °C.

Hydrogen bond breaking in aqueous solutions near the critical point

Abstract The nature of water–anion bonding is examined using X-ray absorption fine structure spectroscopy on a 1m ZnBr 2 /6m NaBr aqueous solution, to near critical conditions. Analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of ZnBr 4 2− , which is the predominant complex at all pressure–temperature conditions investigated. A similar reduction in the hydration shell of waters in the Br − aqua ion was found. Our results indicate that the water–anion and water–water bond breaking mechanisms occurring at high temperatures are essentially the same. This is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolyte solutions.

Analysis of radiation-induced small Cu particle cluster formation in aqueous CuCl2

Synchrotron x-ray radiation is being used extensively as a structure probe to investigate the coordination environment and thus gain insight into the ion–water and ion–ion interactions in aqueous solutions. However, under favorable conditions, there may be instances where the incident x-ray beam can induce oxidation and/or reduction in the solution, thus altering its chemistry. Successive x-ray absorption fine structure spectra, measured in the fluorescence mode from a 55 ppm Cu in CuCl2 aqueous solution, show the formation of copper clusters and their growth as a function of time of irradiation. Initially, the clusters have a nearest neighbor distance of 2.48±0.02 A which, with increase in time of irradiation, increases to 2.55±0.01 A, indicating that the clusters approach the lattice dimensions of bulk copper. Similarly, the Debye–Waller factor of the copper clusters is found to increase by ∼50%–55% over the range of time of irradiation. Analysis of spectra measured in the intermediate time period shows...

Synchrotron x-ray spectroscopy of Eu∕HNO3 aqueous solutions at high temperatures and pressures and Nb-bearing silicate melt phases coexisting with hydrothermal fluids using a modified hydrothermal diamond anvil cell and rail assembly

A modified hydrothermal diamond anvil cell (HDAC) rail assembly has been constructed for making synchrotron x-ray absorption spectroscopy, x-ray fluorescence, and x-ray mapping measurements on fluids or solid phases in contact with hydrothermal fluids up to approximately 900 degrees C and 700 MPa. The diamond anvils of the HDAC are modified by laser milling grooves or holes, for the reduction of attenuation of incident and fluorescent x rays and sample cavities. The modified HDAC rail assembly has flexibility in design for measurement of light elements at low concentrations or heavy elements at trace levels in the sample and the capability to probe minute individual phases of a multiphase fluid-based system using focused x-ray microbeam. The supporting rail allows for uniform translation of the HDAC, rotation and tilt stages, and a focusing mirror, which is used to illuminate the sample for visual observation using a microscope, relative to the direction of the incident x-ray beam. A structure study of Eu(III) aqua ion behavior in high-temperature aqueous solutions and a study of Nb partitioning and coordination in a silicate melt in contact with a hydrothermal fluid are described as applications utilizing the modified HDAC rail assembly.

Relaxation of the structure of simple metal ion complexes in aqueous solutions at up to supercritical conditions

Our previous x-ray absorption fine structure studies of aqueous solutions revealed relaxation of the structure of complexes of Zn2+, Fe2+, La3+, and Yb3+ ions with increasing temperature. These complexes in general exhibit reduction of cation–ligand bond lengths with increasing temperature due to hydrogen bond breaking and loss of water of solvation. Because this results in an overall lowering of the equilibrium state of the complex, we refer to the variation of the structure as relaxation. In the case of M(H2O)mn+ (m=6–9 at room temperature, n=2, 3) aquo ion complexes (M: Zn, Fe, La, Yb), there is a similar reduction in the number of coordinating water molecules with temperature. The relaxation of the structure is shown to result in a lowering of the binding energy per cation–ligand pair of a complex with increasing temperature. A comparison of the rate of structure relaxation with temperature, which is framed in the context of volume thermoelastic constriction, shows this quantity to have a gradual and ...

Steric hindrance and the enhanced stability of light rare-earth elements in hydrothermal fluids

Abstract A series of X-ray absorption spectroscopy (XAS) experiments were made to determine the structure and stability of aqueous REE (La, Nd, Gd, and Yb) chloride complexes to 500 °C and 520 MPa. The REE3+ ions exhibit inner-sphere chloroaqua complexation with a steady increase of chloride coordination with increasing temperature in the 150 to 500 °C range. Furthermore, the degree of chloride coordination of REE3+ inner-sphere chloroaqua complexes decreases significantly from light to heavy REE. These results indicate that steric hindrance drives the reduction of chloride coordination of REE3+ inner-sphere chloroaqua complexes from light to heavy REE. This results in greater stability and preferential transport of light REE3+ over heavy REE3+ ions in saline hydrothermal fluids. Accordingly, the preferential mobility of light REE directly influences the relative abundance of REE in rocks and minerals and thus needs to be considered in geochemical modeling of petrogenetic and ore-forming processes affected by chloride-bearing hydrothermal fluids.

Infrared video microscopy for the study of graptolites and other organic-walled fossils

Organic-walled fossils, such as graptolites and Chitinozoa, show a high degree of transparency to near infrared radiation relative to visible light. Infrared video microscopy (IVM) provides real-time images of the three-dimensional form and internal structure of many specimens that are opaque to visible light. An infrared video camera can be mounted on a biological or petrographic, transmitted-light microscope, and with the addition of a monitor and video printer, images can be viewed and directly printed. With the use of a digital frame-capture system, the IVM images can be digitally stored for analysis, enhancement and printing. As compared with scanning electron microscopy, which only reveals the external form, IVM shows such features as fusellar growth bands and internal septa of graptolites and the prosome structure of Chitinozoa. This method eliminates the need for chemical clearing of specimens for study in visible light, which damages the surface texture and commonly renders specimens too brittle for further manipulation. Infrared video microscopy is potentially applicable to the study of any organic-walled fossils and to the petrographic study of sedimentary organic matter.