Collaborative Research: Redox Ratios by Fe-XANES
Of all the common elements in the Earth's crust, iron is the only element that occurs in multiple valence states; it can be found as a pure Fe(0) metal, as Fe(II), or as Fe(III). The valence state of iron thus reflects the amount of oxygen that was present in the environment at the time the mineral crystallized. The amount of oxygen in turn affects how a magma may crystallize: which minerals will form, and in what order. For this reason, measuring the oxidation state of Fe is extremely important in all rock types in which the minerals occur in equilibrium. The valence state of Fe thus serves as a recorder of the evolution of oxygen in the Earth's interior and on its surface. It is also useful in identifying geological formations of economic interest (for example, garnets in diamond-bearing deposits are usually oxidized). The ability to measure Fe valence state is also useful in understanding chemical reactions that control industrial processing of minerals, chemical and metallurgical engineering, recycling of materials, and even the study of health effects and mineralogy.
Historically, it has been very difficult to measure the valence state of Fe in minerals quantitatively. Traditional wet chemistry methods have been used for a half-century, but they require large masses of sample (>1 g) and these are difficult to prepare from rocks that may contain intimate mixtures of fine-grained minerals. MÃƒÂ¶ssbauer spectroscopy is another bulk technique that has more recently been used for measurements of Fe, but it also requires >20 mg of sample. Microfocused X-ray Absorption Near-Edge Spectroscopy (XANES) presents a promising technology for making Fe3+/total Fe measurements on small spots (8-30 microns) on mineral grains. This technique utilizes the observation that the energy of the Fe K absorption edge and the absorption pre-edge feature found ~15-20 eV below the edge are very sensitive to the oxidation state of Fe. Past measurements of Fe3+/total Fe by this technique have been only semi-quantitative, but recent upgrades at the hard x-ray microprobe beamline x26a at the National Synchrotron Light Source (higher beam intensity and improved energy resolution available from a Si(311) monochromator) have remedied these issues. Thus, the ambitious goal of this proposal is to develop the XANES technique for use as a quantitative microprobe method for analysis of Fe(II) and Fe(III) in minerals.