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Critical to interpreting platinum chemical speciation using X-ray absorption spectroscopy (XAS) is the availability of reference spectra of compounds with known Pt redox and coordination. Here we compare different techniques for Pt LIII-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectral regions for a large set of Pt-O-Cl-S reference compounds of known structures. The measurements were conducted in HERFD (high-energy resolution fluorescence detection, high-resolution or HR) mode, as well as in two conventional modes such as transmission (TR) and nominal-resolution total fluorescence yield (TFY or NR). Samples analyzed here included Pt0 (TR), PtIIS (HR), PtIVS2 (TR), K2PtIICl4 (HR + TR), K2PtIVCl6 (HR + TR), PtIVO2 (HR + TR), C6H12N2O4PtII (HR + TR), and aqueous solutions of K2PtIICl4 and H2PtIVCl6 (NR + TR), as well as (NH4)2PtIV(S5)3 (HR + TR). XANES spectra in HERFD mode offer a better energy resolution than in conventional modes, allowing a more accurate identification of Pt redox state and coordination geometry. EXAFS spectra in all three modes for a given compound yield identical within errors values of Pt-neighbor interatomic distances and mean square relative displacement (MSRD, σ2) parameters. In contrast, both TR and NR spectra on the one hand and HR spectra on the other hand yield distinct amplitude reduction factor (S02) values, 0.76 ± 0.04 and 0.99 ± 0.07 (1 standard error), respectively. This study contributes to the development of an open-access XAS database SSHADE. [ABSTRACT FROM AUTHOR]

Gold mobilization, transfer, and concentration in the Earth's crust are controlled by hydrothermal sulfur- and chloride-bearing fluids. Yet the exact chemical identity, structure, and stability of Au-bearing species and, in particular, the respective contributions of the sulfide (HS−) and trisulfur ion ( S 3 ∙ − $\text{S}_{3}^{\bullet-}$) ligands to Au transport lack direct in situ evidence. Here we employed high energy resolution fluorescence detection X‑ray absorption spectroscopy (HERFD-XAS) on aqueous sulfate/sulfide/ S 3 ∙ − $\text{S}_{3}^{\bullet-}$ -bearing solutions at typical hydrothermal temperatures and pressures (T = 350 °C, P = 600 bar) to reveal differences in dissolved Au spectral signatures indicative of contrasting fluid-phase Au speciation as a function of acidity and redox conditions. Combined with in situ Au solubility measurements and quantum-chemical and thermodynamic modeling, our spectroscopic data provide direct evidence for the Au(HS)S͞3 and Au(HS)S͞2 complexes predominant at acidic-to-neutral and alkaline conditions, respectively. Our findings thus directly confirm a recent speciation scheme for Au in aqueous S-bearing fluids established using less direct methods, and highlight an important role of the trisulfur ion in gold mobilization and concentration in hydrothermal-magmatic deposits associated with subduction zones. More generally, our results show that HERFD-XAS enables the identification of structural and coordination features in metal complexes virtually unresolvable using classical XAS techniques. By avoiding limitations of less direct techniques, our integrated high-resolution spectroscopic approach opens perspectives for studies of the speciation and solubility of gold and other metals in high T-P fluids, and potentially silicate melts, inaccessible to direct observation in nature. [ABSTRACT FROM AUTHOR]

Understanding the behavior of halogens (Cl, Br, and I) in subduction zones is critical to constrain the recycling of trace elements and metals, and to quantify the halogen fluxes to the atmosphere via volcanic degassing. Here, the partitioning of bromine between coexisting aqueous fluids and hydrous granitic melts and its speciation in slab-derived fluids have been investigated in situ up to 840 °C and 2.2 GPa by X-ray fluorescence (SXRF) and absorption (XANES and EXAFS) spectroscopy in hydrothermal diamond-anvil cells. The partition coefficients Df/mBr range from 15.3 ± 1.0 to 2.0 ± 0.1, indicating the preferential uptake of Br by aqueous fluids at all investigated conditions. EXAFS analysis further evidences a gradual evolution of Br speciation from hydrated Br ions [Br(H2O)6]- in slab dehydration fluids to more complex structures invoving both Na ions and water molecules, [BrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greather depth (> 200 km). In dense fluids containing 60 wt % dissolved alkali-silicates and in hydrous Na2Si2O5 melts (10 wt % H2O), Br is found in a "salt-like" structure involving 6 nearest Na ions and several next-nearest O neighbors that are either from water molecules or the tetrahedral silicate network. Bromine (and likely chlorine and iodine) complexation with alkalis is thus an efficient mechanism for the mobilization and transport of halogens by hydrous silicate melts and supercritical fluids, which can carry high amounts of Br, up to the 1000 ppm level. Overall, our results suggest that both shallow dehydration fluids and deeper silicate-bearing fluids efficiently remove halogens from the slab in the sub-arc region, thus controling an efficient recycling of halogens in subduction zones. [ABSTRACT FROM AUTHOR]

Abstract: Despite the common belief that AuI complexes with hydrogen sulfide ligands (H2S/HS−) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous AuI–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local atomic structure around the dissolved metal in S–NaOH–Na2SO4–H2SO4 aqueous solutions were characterized at temperatures 200–450°C and pressures 300–600bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)2 − is the dominant Au species in neutral-to-basic solutions (5.5⩽pH⩽8.5; H2O–S–NaOH) over a wide range of sulfur concentrations (0.2<ΣS<3.6mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur atoms are in a linear geometry around AuI at an average distance of 2.29±0.01Å. At acidic conditions (1.5⩽pH⩽5.0; H2O–S–Na2SO4–H2SO4), the Au atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)0 complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H2S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H2S concentrations higher than 0.1–0.2mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of AuI complexes with these ligands (e.g., AuHS(SO2)0, Au(HS2O3)2 −, Au(HS n )2 −) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth. [Copyright &y& Elsevier]

Abstract: The solubility of bismuth oxide (α-Bi2O3(s); bismite) in near-neutral sodium perchlorate solutions at 65 and 80°C, and pure water from 150 to 600°C, P Sat to 800bar was studied using various batch-reactor techniques and in situ XAS spectroscopy. The solubility of Bi2O3(s) follows a similar trend to Sb2O3(s) (senarmontite), which has been interpreted in terms of a neutral Sb(OH)3(aq) complex. Thus a similar neutral complex, Bi(OH)3(aq), is inferred for Bi. XANES spectroscopy confirms that the Bi(OH)3(aq) complex carries a stereochemically active lone electron pair, and EXAFS data suggest that the geometry of the complex changes little over the temperature range 380–610°C at 800bar, with three oxygen neighbors at ∼2.08Å. The solubility data obtained in this study are used in conjunction with thermodynamic properties for α-Bi2O3(s) to obtain thermodynamic parameters for Bi(OH)3(aq) within the framework of the revised Helgeson–Kirkham–Flowers (HKF) equation of state. Speciation calculations using these new properties indicate that, similarly to arsenic and antimony, bismuth is transported predominantly as a neutral hydroxide complex in a wide range of temperature, pressure, and fluid compositions. In contrast to arsenic and antimony, bismuth is much less soluble in typical hydrothermal fluids in the form of hydroxide complexes, and high temperatures (⩾400°C) are required for significant Bi transport by aqueous fluids. These results are consistent with the common association between Bi mineralization and magmatism. [Copyright &y& Elsevier]

Despite the importance of phototrophic biofilms in metal cycling in freshwater systems, metal isotope fractionation linked to metal adsorption and uptake by biofilm remains very poorly constrained. Here, copper isotope fractionation by a mature phototrophic biofilm during Cu surface adsorption and incorporation was studied in batch reactor (BR) and open drip flow reactor (DFR) systems at ambient conditions. X-ray Absorption Spectroscopy (both Near Edge Structure, XANES, and Extended Fine Structure, EXAFS) at Cu K-edge of the biofilm after its interaction with Cu in BR experiments allowed characterizing the molecular structure of assimilated Cu and quantifying the degree of Cu II to Cu I reduction linked to Cu assimilation. For both BR and DFR experiments, Cu adsorption caused enrichment in heavy isotope at the surface of the biofilm relative to the aqueous solution, with an apparent enrichment factor for the adsorption process, ε 65 Cu ads , of +1.1 ± 0.3‰. In contrast, the isotope enrichment factor during copper incorporation into the biofilm (ε 65 Cu inc ) was highly variable, ranging from −0.6 to +0.8‰. This variability of the ε 65 Cu inc value was likely controlled by Cu cellular uptake via different transport pathways resulting in contrasting fractionation. Specifically, the Cu II storage induced enrichment in heavy isotope, whereas the toxicity response of the biofilm to Cu exposure resulted in reduction of Cu II to Cu I , thus yielding the biofilm enrichment in light isotope. EXAFS analyses suggested that a major part of the Cu assimilated by the biofilm is bound to 5.1 ± 0.3 oxygen or nitrogen atoms, with a small proportion of Cu linked to sulfur atoms (N S < 0.6) of sulfhydryl groups. XANES analyses showed that the proportion of Cu II vs Cu I , compared to the initial Cu II /Cu I ratio, decreased by 14% after the first hour of reaction and by 6% after 96 h of reaction. The value of ε 65 Cu inc of the biofilm exhibited a similar trend over time of exposure. Our study demonstrates the complexity of biological processes associated with live phototrophic biofilms, which produce large and contrasting isotope fractionations following rather small Cu redox and speciation changes during uptake, storage or release of the metal, i.e. , favoring heavy isotopes during complexation with carboxylate ligands and light isotopes during reduction of Cu II -O/N to Cu I -sulfhydryl moieties. [ABSTRACT FROM AUTHOR]