RUI: Collaborative Research: Redox Ratios by Fe-XANES

在地壳中的所有常见元素中，铁是唯一以多价态存在的元素;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。 氧气的含量反过来又会影响岩浆结晶的方式：哪些矿物质会形成，以及形成的顺序。 因此，测量铁的氧化态在矿物处于平衡状态的所有岩石类型中都非常重要。 因此，铁的价态记录了地球内部和表面氧的演化。 它还有助于查明具有经济价值的地质构造（例如，含钻石矿床中的石榴石通常被氧化）。 测量铁的价态的能力也有助于了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，甚至是健康影响和矿物学的研究。历史上，定量测量矿物中铁的价态非常困难。 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 g），并且很难从可能含有细粒矿物的紧密混合物的岩石中制备。 穆斯堡尔谱是另一种最近被用于测量铁的散装技术，但它也需要20毫克的样品。 微聚焦X射线吸收近边光谱（XANES）提出了一种有前途的技术，用于在矿物颗粒上的小斑点（8-30微米）上进行Fe 3 +/总Fe测量。该技术利用了以下观察结果：Fe K吸收边的能量和边缘以下约15-20 eV处发现的吸收边前特征对Fe的氧化态非常敏感。 过去通过该技术测量Fe 3 +/总Fe仅为半定量，但最近在国家同步加速器光源的硬X射线微探针光束线x26 a处的升级（更高的光束强度和改进的能量分辨率可从Si（311）单色器获得）已经补救了这些问题。 因此，该提案的宏伟目标是开发XANES技术，用作矿物中Fe（II）和Fe（III）的定量微探针分析方法。

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.