Collaborative Research: Redox Ratios by Fe-XANES

合作研究：Fe-XANES 的氧化还原比

• 批准号: 0809253
• 负责人: Antonio Lanzirotti
• 金额: $ 8.13万
• 依托单位: University of Chicago
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-07-01 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809253&HistoricalAwards=false
• 关键词: Collaborative; Research; Redox; Ratios; Fe

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.

在地壳中的所有常见元素中，铁是唯一以多种价态存在的元素。它可以以纯 Fe(0) 金属、Fe(II) 或 Fe(III) 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。氧气的含量反过来会影响岩浆的结晶方式：会形成哪些矿物质以及以什么顺序形成。因此，测量铁的氧化态对于矿物处于平衡状态的所有岩石类型都极其重要。因此，Fe 的价态可以作为地球内部及其表面氧气演化的记录器。它还可用于识别具有经济利益的地质构造（例如，含钻石矿床中的石榴石通常会被氧化）。测量 Fe 价态的能力对于理解控制矿物工业加工、化学和冶金工程、材料回收、甚至健康影响和矿物学研究的化学反应也很有用。历史上，定量测量矿物中 Fe 的价态一直非常困难。传统的湿化学方法已经使用了半个世纪，但它们需要大量样品（1 克），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这些样品。穆斯堡尔光谱是最近用于测量 Fe 的另一种批量技术，但它也需要 20 毫克样品。微聚焦 X 射线吸收近边光谱 (XANES) 是一种很有前景的技术，可用于在矿物颗粒上的小点（8-30 微米）上进行 Fe3+/总铁测量。该技术利用了以下观察结果：Fe K 吸收边的能量和吸收前边特征（在边下方约 15-20 eV 处发现）对 Fe 的氧化态非常敏感。过去通过该技术对 Fe3+/总 Fe 的测量仅是半定量的，但最近国家同步加速器光源的硬 X 射线微探针光束线 x26a 的升级（Si(311) 单色仪提供更高的光束强度和改进的能量分辨率）已经解决了这些问题。因此，该提案的宏伟目标是开发 XANES 技术，用作分析矿物中 Fe(II) 和 Fe(III) 的定量微探针方法。