Rinat Gabitov

Radioiodine (129I) poses a long-term environmental and health risk due to its high mobility, solubility, radiotoxicity, and long half-life. Owing to chemical similarities between iodine species and other anions accommodated in apatite, this mineral represents a promising host for radioiodine immobilization. Nevertheless, the mechanisms of iodine incorporation into the apatite structure remain insufficiently understood. In this study, we evaluated the immobilization of iodate by Cl-OH and hydroxyapatite under hydrothermal conditions. Experimental solids were characterized using electron microprobe, scanning electron microscopy, X-ray diffraction, synchrotron X-ray absorption spectroscopy, and atom probe tomography. Experimental solutions were analyzed by ultraviolet–visible spectrophotometry and ion chromatography. The highest iodate concentration measured in apatite was 6.0 wt.% when a 0.1 M NaIO₃ solution was used as the crystal growth medium. However, the ratio of iodate incorporated into the solid relative to its concentration in solution increased with decreasing aqueous iodate concentration, indicating enhanced compatibility of iodate within apatite at ppm-level aqueous concentrations. These results suggest that iodate may substitute for both phosphate and hydroxyl (or chloride) in the apatite structure.

This study aims to evaluate relationships between elemental signatures in calcite and the crystallographic orientation of its planes. The ability of calcite (a widespread calcium carbonate mineral) to entrap various trace and minor elements in its structure is the foundation of multiple methods (also called proxies) to reconstruct paleoenvironment conditions (e.g., temperature, pH, and marine chemistry). Although several element-to-calcium ratios (E/Ca) are routinely measured in marine carbonates and are widely used in paleoclimate studies, some of the controls on the incorporation of these elements are still unclear. Here, we examine the effect of crystallography on (E/Ca)_calcite by growing thin layers of calcite on differently oriented Iceland Spar substrates immersed in modified seawater solution. Newly grown calcite layers were examined with Laser Ablation Inductivity–Coupled Plasma Mass Spectrometry (LA-ICP-MS), Backscattered Electron Imaging (BSE), and Energy Dispersive X-ray Spectroscopy (EDS). We propose that the crystallographic orientation might slightly influence the incorporation of lithium (Li), sodium (Na), magnesium (Mg), sulfur (S), and barium (Ba) into the studied calcite faces and have no impact on the incorporation of boron (B), potassium (K), and strontium (Sr) at least under the conditions of our experiment.

The work presented sought to determine the effects of Mg/Ca ratios in solution have on Mg partitioning (K^Mg) between precipitated abiotic low-Mg calcite and solution. Experiments were set up so that Mg/Ca in precipitated abiotic calcite would match the Mg/Ca in planktonic foraminifera. This research intended to investigate the effect of Mg/Ca_(Fluid) on K^Mg when the molar value of Mg/Ca_(Fluid) was below 0.5, which is below the previously reported Mg/Ca range. The values of pH, salinity, and aqueous Mg/Ca were monitored during calcite precipitation, and Mg/Ca of calcite was determined at the end of experiments. Partition coefficients of Mg were evaluated as a ratio of Mg/Ca in calcite to the averaged ratio of aqueous Mg/Ca for each experiment.

Transformation of aragonite, a mineral phase metastable at Earth’s surface, to calcite widely occurs in both sedimentary and metamorphic systems with the presence of an aqueous phase. The transformation process can affect geochemical signatures of aragonite (a protolith). This study focused on quantification of the retention of Mg/Ca and Sr/Ca ratios, and δ¹⁸O during the transformation process as well as evaluation of the transformation rate. To investigate the effect of transformation from aragonite to calcite on elemental and stable isotope ratios, we conducted a series of experiments in NaCl solutions at temperatures between 120 and 184 °C. Two additional experiments at 250 °C were conducted to estimate the transformation rate of aragonite to calcite. Protolith materials consist of (1) synthetic (Mg; Sr-bearing or non-Mg; Sr bearing) needle-shaped microcrystals of aragonite (100 µm in size) of natural aragonite. X-ray diffraction (XRD) showed that microcrystals successfully transformed to calcite within 30 h and scanning electron microscopy (SEM) yielded a change in the crystal size to >10 µm in rhombohedral shape. Electron backscatter diffraction (EBSD) of the larger aragonite chips showed that transformation to randomly oriented calcite occurred at the rims and along the cracks while the core retained an aragonite crystal structure. Isotope-ratio mass spectrometry (IRMS) analyses showed that calcite δ¹⁸O was controlled by temperature and δ¹⁸O of the solution. The obtained calibration curve of isotope fractionation factor versus temperature is consistent with other studies. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses showed that calcite partially or completely retained Mg/Ca and Sr/Ca ratios through the transformation.

This study is focused on the mineralogical, chemical, and isotopic characterization of pyrites from the rocks of the Bazhenov Formation (Upper Jurassic–Lower Cretaceous organic-rich shales, Western Siberia, Russia). Scanning electron microscopy (SEM) revealed pyrites of different morphologies: small and large framboids, small crystals, and large euhedral crystals; all morphotypes were usually combined into aggregates. Isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (SIMS) showed that small framboids and microcrystalline pyrite are isotopically light, with δ³⁴SCDT varying from −55 to −20‰. Large framboids and euhedral crystals of pyrite are isotopically heavy with δ³⁴SCDT up to +26‰. Both morphology and δ³⁴S were suggested to be controlled by the redox conditions and sedimentation regime. The abundance of small framboids suggests that pyrite sedimentation occurred under anoxic conditions; the presence of the large framboids and euhedral crystals of pyrite suggest the accumulation of sediments occurred at suboxic conditions, possibly in the presence of oxygen.

Studies on incorporation of radionuclides into the crystal structures of phosphate minerals strongly indicate that uranium and its mobile fission products can be efficiently immobilized through uptake from aqueous solution by formation of phosphate and carbonate minerals. Aiming at development of a new engineered backfill material, we have investigated uptake of uranium at a range of temperatures similar to those expected at waste repository sites, where thermal peak of the waste package (heated by radioactive decay) is ~300°C (Greenburg and Wen 2013). The available literature data on uranium uptake by phosphates are limited to ambient temperatures (e.g. Arey et al. 1999), and to our best knowledge no experimental studies on uranium uptake by phosphates and carbonate at hydrothermal conditions have been performed.Experiments were conducted in the autoclaves at saturated water pressure and 200-350°C, where metastable phosphate (brushite) and carbonate (aragonite) were transformed to apatite/monetite and calcite in NaCl solutions. Uranium was introduced into autoclave in separate tubes in the forms of U3O8 and UO3 in experiments at reduced and oxidized conditions. Oxidation state of dissolved uranium (U4+ or U6+) was controlled by addition of solid redox buffers into autoclaves.X-ray diffraction (XRD) and backscattered electron diffraction (EBSD) of crystalline products allowed estimation of mineral transformation rate. Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS allowed obtaining uranium content in crystals and its concentration in co-existed solution. Partition coefficient (D) of uranium was calculated as the ratio of uranium content in the solid to uranium concentration in the solution. Selected solids were examined with synchrotron-based X-ray absorption spectroscopy (XAS).Overall, our results showed: 1) brushite transforms to monetite and apatite mixture during 6 days, but up to 1 month is required for complete transformation to apatite; 2) mineralogy of the final phase (monetite or hydroxyapatite) depends on ionic strength of the solution (confirmed by thermodynamic calculations); 3) uranium is compatible with phosphate and carbonate minerals, where D could be as high as 1000; 4) uptake of U4+ by calcite is higher than that of U6+ by up to a factor of 100; 5) uranium incorporates into calcite structure as U6+ at oxidized conditions. Additional analyses are pending and results will be presented at EGU meeting. ReferencesArey J.S., Seaman J.C., and Bertsch P.M. (1999) Environ. Sci. Technol.  33, 337-342.Greenburg H.R. and Wen J. (2013) LLNL-TR639869-DRAFT, 38.

Crystal growth rate has not been sufficiently explored to understand element partitioning between calcite and seawater solutions. We investigated the uptake of Li, B, Mg, Sr, and Ba by Mg-bearing calcite slowly grown on a calcite cleavage fragment. Experiments were conducted by elevating the alkalinity of an artificial seawater solution. Growth rates were evaluated by addition of lanthanum spike. At the end of each experiment, cleavage fragments were extracted and examined with micro-Raman spectroscopy, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) using depth profiling technique. Distribution of Li, B, Mg, Sr, and Ba in calcite overgrowth as well as partition coefficients of those elements were evaluated.

This study is focused on mineralogical and chemical characterization of an authigenic carbonate rock (crust) collected at a recently discovered cold seep on the US North Atlantic continental margin. X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the carbonate rock is composed of microcrystalline aragonite cement, white acicular aragonite crystals (AcAr), equant quartz crystals, small microcrystalline aluminosilicates, and trace amounts of iron sulfide microcrystals. Element/calcium ratios were measured with laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) using a calcite standard, which was prepared by annealing USGS certified carbonate powder (MACS-3). The occurrence of microscopic, non-carbonate inclusions precluded evaluation of trace elements in the aragonite cement, but allowed for in situ analysis of AcAr crystals. Carbon and oxygen isotopes were analyzed via isotope ratio mass spectrometry (IRMS) and expressed as δ¹³C and δ¹⁸O. Low δ¹³C values suggest that aragonite grew as a result of anaerobic oxidation of methane and observed δ¹⁸O values indicate that the temperature of aragonite crystallization was 1.7–1.9 °C.