Karen M. Love

Identification of sulfate in natural carbonates by x-ray absorption spectroscopy

Abstract We have analyzed sulfur K-edge X-ray absorption near-edge structures (XANES) and demonstrated, with a high degree of certainty, that sulfur is present in a variety of biogenic and diagenetic carbonates as sulfate. The sulfate is clearly not in the form of gypsum or anhydrite inclusions, and there is good evidence that the sulfate substitutes for carbonate. This finding validates systematic investigations of sulfate distribution in modern and ancient carbonate minerals. Further, it documents that carbonate sedimentation is a sink for marine sulfate, accounting for perhaps 15% of the annual sulfur sedimentary depositional flux.

Dissolution Kinetics Of CaCO3 In Common Laboratory Solvents

ABSTRACT The dissolution of calcium carbonate with both organic and aqueous solvents was examined quantitatively and kinetically. Neither ethanol and nor acetone at 25°C dissolved appreciable aragonite or calcite, even after nearly ten days. Bleach (sodium hypochlorite) was a much less effective solvent than deionized water, producing little or no net calcium in solutions in contact with calcium carbonate. Although exposure to sodium hydroxide solutions also yielded relatively small concentrations of calcium in the fluid, characterization of the exposed carbonate grains by X-ray powder diffraction and by light microscopy revealed extensive replacement of the CaCO3 by Ca(OH)2, calcium hydroxide (portlandite). A solution of AlconoxTM, a commercial laboratory nionic detergent, was about half again as potent as deionized water, and hydrogen peroxide proved to be many times more corrosive of calcium carbonate than deionized water. These results are consistent with published and anecdotal observations of the effects of laboratory preparation protocols on calcium carbonate. Our quantitative data permit the researcher to assess the potential for a treatment procedure using any of these solutions to compromise the integrity of a sample of calcium carbonate.

Discrimination of sulfate from sulfide in carbonates by electron probe microanalysis

Using standards and samples previously analyzed by X-ray Absorption Spectroscopy (XAS), and additional materials, we have demonstrated that the valence of sulfur in a carbonate thin section can be speciated reasonably well with an electron microprobe. The wavelength of sulfur K-α radiation varies slightly and systematically with valence; this can be detected as a shift in the peak position (Bragg angle of the crystal) in the wavelength dispersive spectrometer (WDS). In a series of model compounds, sulfate (S6+) and sulfite (S4+) were easily distinguished from sulfur (S80), disulfide (S22−), and sulfide (S2−). Further distinction within these two groups depends on the S concentration and instrument sensitivity; beam damage to the sample is a limiting factor. Microprobe analysis of carbonate samples previously studied by XAS yielded S speciation in the sulfate-sulfite grouping. This is consistent with the earlier, high-resolution, XAS finding of sulfate substituting for carbonate. Speciation of sulfur using the microprobe permits spatially resolved analysis of the S intimately associated with the carbonate material. The technique is extending our assessment of the form of S in a variety of marine bio-carbonates and reservoir dolomites. Determination of sulfur valence also constrains Eh conditions in the solution at the time of mineral formation or diagenesis.