Evidence of a Stable Uranyl Site in Ancient Organic-Rich Calcite

Abstract

The mechanism of uranium (U) incorporation into calcite (calcium carbonate) is of fundamental importance to the fate and transport of U at the surface and in the shallow subsurface and has implications for (a) the accuracy of U-Pb and U-series isotope ratio methods used to determine the ages of ancient deposits and (b) potential remediation strategies based on sequestration of U in the subsurface. Extended X-ray absorption fine structure (EXAFS) spectroscopy is uniquely suited to the study of U-calcite systems. The sensitivity of the EXAFS spectrum to the local atomic Ca coordination about U(VI) in the calcite structure results in an increase in the number and amplitude of Ca signals as the U(VI) becomes more ordered within the crystal structure. Our X-ray microprobe (10-microm) measurements of an ancient 298 million-year-old organic-rich calcite (calcrete) clearly revealed three coordination shells of Ca atoms, defining a well-ordered calcite structure about uranyl to a distance of approximately 6.5 angstroms. These results indicate that uranyl is incorporated at the Ca2+ site in calcite and that the uranyl environment may evolve over long time scales, becoming more calcite-like and more stable for long-term sequestration of uranium. These results therefore validate U-related dating methods and show that calcite can be effective at sequestering U in vadose zone sediments.

Calcite and dolomite in intrusive carbonatites. II. Trace-element variations

“…In the range of conditions where both CeO 2 and Pb carbonates are stable (i.e., Eh > 0 and mildly alkaline pH), U is oxidized and forms uranylcarbonate complexes (Ondruš et al 2003), suggesting that (UO 2 ) 2+ is more readily incorporated in calcite than U 4+ prevalent in the igneous environment. This conclusion is consistent with high concentrations of structurally bound U 6+ in some sedimentary calcite (Kelly et al 2006).…”

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

“…Recent EXAFS studies have characterized aqueous uranyl carbonate species (Allen et al, 1995); aqueous uranyl citrate species (Bailey et al, 2005); uranyl carbonates adsorbed onto hematite (Bargar et al, 2000), goethite, and natural sediments in the presence of calcium (Duff and Amrhein, 1996;Duff et al, 1997); and uranyl adsorbed onto ferrihydrite and silica gels (Reich et al, 1998). Uranyl incorporation into calcium carbonate and surface adsorption during calcite growth have also been studied by EXAFS and X-ray surface scattering (Reeder et al, 2000(Reeder et al, , 2001Elzinga et al, 2004), and a few studies have addressed uranium naturally incorporated into calcite (Sturchio et al, 1998;Kelly et al, 2003Kelly et al, , 2006.…”

Uranium isotope fractionation during coprecipitation with aragonite and calcite

“…U concentrations and isotope compositions in carbonate rocks could be a very useful paleoredox proxy because oxidized U(VI) is readily incorporated into calcium carbonate mineral structures during carbonate precipitation(i.e., "coprecipitation") in relatively high concentrations of 0.1 to 10 ppm (Chung and Swart, 1990;Kelly et al, 2003Kelly et al, , 2006Reeder et al, 2000). Additionally, the long residence time of U (>10 5 yr; Dunk et al, 2002) suggests that, unlike Ce, U might provide globally integrated paleoredox information.…”