In situ synchrotron X-ray diffraction of Fe oxide transformations in aqueous solutions

矿物赤铁矿（Fe2O3）具有重要的经济意义，因为它是提取铁的主要矿物储备，占世界金属消费量的90%。此外，赤铁矿在土壤中的形成具有最高的环境意义，因为赤铁矿纳米颗粒可以附着在土壤颗粒表面并改变其隔离有害金属的能力。赤铁矿在技术上也被用于从磁性数据存储到色素沉着的各种应用中，并且它正在作为太阳能电池中的水分解剂进行测试。然而，尽管赤铁矿在地球表面无处不在，赤铁矿结晶的化学反应仍然知之甚少。本研究将探索一种新的假设，即从水溶液中成核的第一个赤铁矿晶体在化学和物理上与理想的赤铁矿不同，这不仅是因为它们的纳米级粒度，而且因为晶体结构中缺失了多达三分之一的铁原子。先前的研究表明，这些铁缺陷可以显著改变赤铁矿的磁性、颜色饱和度和化学反应性。在这项研究中，研究人员将探索铁缺陷与赤铁矿物理性质之间的关系，他们将确定铁缺陷丰度与天然赤铁矿结晶的地下水的温度、pH和氧含量之间的联系。研究人员假设，当赤铁矿（Fe2O3）由水合铁和针铁矿在水溶液中形成时，赤铁矿首先成核为高度缺铁的含水纳米晶体。这些有缺陷的前体相是亚稳态的，随着时间的推移可能退火为端元赤铁矿。研究人员根据他们最近在氯化铁溶液中结晶顺序的观察得出这些推论。这项工作揭示了“水赤铁矿”的初始阶段，其化学式接近于由前体akagansamuite形成的FeOOH。研究人员将确定天然赤铁矿更常见的结晶途径——从先前的水合铁和针铁矿——是否经历类似的多步骤反应途径。与基于粉末干燥加热或批量反应混合物“前后”表征的方法相反，研究人员将利用基于同步加速器的新型技术，使他们能够以非常高的时间分辨率捕获25至200摄氏度的矿物-流体混合物的x射线衍射模式。衍射数据符合Rietveld分析，将产生高质量的细胞参数、原子位置和占位率。除了在原子尺度上精确地指出赤铁矿生长的演化，实验还将产生动力学模型，从中他们可以提取铁（氢）氧化物成核和生长的强大活化能，作为ph的函数。研究人员将把这些实验与详细的显微镜和光谱检查结合起来，对“水赤铁矿”的自然和历史标本进行详细的显微镜和光谱检查。他们将进行密度泛函计算，以衡量含水赤铁矿相的相对稳定性，并将化学性质与粒度联系起来。

The mineral hematite (Fe2O3) is of critical economic importance, as it serves as the primary mineral reserve for the extraction of iron, which accounts for 90% of worldwide metal consumption. In addition, hematite formation in soils is of the highest environmental significance, because hematite nanoparticles can attach to the surfaces of soil particles and modify their ability to sequester harmful metals. Hematite also is used technologically in applications ranging from magnetic data storage to pigmentation, and it is being tested as an agent for water-splitting in solar cells. However, despite the ubiquity of hematite at Earth's surface, the chemical reactions by which hematite crystallizes are still poorly understood. This study will explore a novel hypothesis that the first crystals of hematite to nucleate from aqueous solutions are chemically and physically distinct from ideal hematite, not only because of their nanometer-scale particle size but because up to one-third of the iron atoms are missing from the crystal structure. Previous studies have shown that these iron defects can dramatically alter the magnetic properties, color saturation, and chemical reactivity of hematite. In this study, the investigators will explore the relation between iron defects and the physical properties of hematite, and they will determine the connection between the abundance of defects and the temperature, pH, and oxygen content of the groundwaters from which natural hematite crystallizes.The investigators hypothesize that when hematite (Fe2O3) forms from ferrihydrite and goethite in aqueous solutions, the hematite first nucleates as highly iron-deficient, hydrous nanocrystals. These defective precursor phases are metastable and may anneal to endmember hematite with time. The investigators base these inferences on their recent observations of crystallization sequences in ferric chloride solutions. That work revealed incipient phases of "hydrohematite" with a chemical formula close to FeOOH forming from precursor akaganéite. The investigators will determine whether the more common crystallization pathways for natural hematite - from antecedent ferrihydrite and goethite - experience an analogous multi-step reaction route. In contrast to approaches based on dry heating of powders or on "before-and-after" characterization of batch-reacted mixtures, the investigators will exploit novel synchrotron-based techniques that allow them to capture X-ray diffraction patterns of mineral-fluid mixtures from 25 to 200 degree C with very high time resolution. The diffraction data are amenable to Rietveld analysis and will yield high-quality cell parameters, atom positions, and occupancies. In addition to pinpointing the evolution of hematite growth at the atomic scale, the experiments will yield kinetic models from which they can extract robust activation energies for the nucleation and growth of iron (hydr)oxides as a function of pH. The investigators will couple these experiments with detailed microscopic and spectroscopic examinations of natural and historic specimens of "hydrohematite", and they will perform density functional calculations to gauge the relative stabilities of hydrous hematite phases and to relate chemical properties to particle size.