RUI: Collaborative Research: Redox Ratios by Fe-XANES

在地壳中的所有常见元素中，铁是唯一以多种价态出现的元素；它可以以纯Fe(0)金属、Fe(II)或Fe(III)的形式存在。因此，铁的价态反映了矿物结晶时环境中存在的氧气量。氧气的数量反过来影响岩浆的结晶方式：将形成哪些矿物，以及形成的顺序。因此，在矿物平衡存在的所有岩石类型中，测量铁的氧化态是极其重要的。因此，铁的价态是地球内部和表面氧气演化的记录器。它还有助于识别具有经济意义的地质构造(例如，含钻石矿床中的石榴石通常被氧化)。测量铁价状态的能力也有助于理解控制矿物工业加工、化学和冶金工程、材料回收的化学反应，甚至是对健康影响和矿物学的研究。从历史上看，定量测量矿物中铁的价态一直是非常困难的。传统的湿化学方法已经使用了半个世纪，但它们需要大量的样品(1克)，而且很难从可能包含细粒矿物的亲密混合物的岩石中制备。穆斯堡尔光谱分析是最近用于铁测量的另一种大宗技术，但它也需要20毫克的样品。微聚焦X射线吸收近边能谱(XANES)是一种在矿物颗粒小光点(8-30微米)上测量Fe3/总铁的很有前途的技术。这项技术利用了Fe-K吸收边的能量和在边缘以下15-20 eV的吸收前缘特征对Fe的氧化态非常敏感的观测。过去用这种技术测量Fe3/总铁只是半定量的，但最近国家同步辐射光源的硬X射线微探头光束线X26a的升级(从Si(311)单色仪可以获得更高的光束强度和更高的能量分辨率)已经解决了这些问题。因此，这项建议的雄心勃勃的目标是开发XANES技术，用作分析矿物中Fe(II)和Fe(III)的定量微探针方法。

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. Mössbauer 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.