Structural controls on Fe oxide formation: A crystallographic analysis of the growth of hematite versus goethite

追溯到最早的原始人，赤铁矿和针铁矿(都是铁氧化物)为我们提供了持久的、无毒的红色、黄色和棕色颜料，今天这些矿物被合成了数百万吨。它们还具有关键的经济重要性，因为它们是提炼铁的主要储备，而铁占全球金属消费量的90%。此外，它们还可以控制土壤去除地下水污染物的能力，因为它们经常以涂层的形式形成在其他土壤矿物上，如石英和长石。即使只是一种微小的贴面，它们也可以吸引和去除有毒金属，否则有毒金属不会与石英或长石主体发生反应。尽管它们具有经济和环境意义，但地质学家们仍在争论它们是如何形成的，以及什么机制决定了赤铁矿或针铁矿中的哪一个可能结晶。了解这一形成过程很重要，因为一些科学家利用赤铁矿和针铁矿的比例来推断形成古代土壤的气候条件。该项目致力于确定决定赤铁矿或针铁矿将在自然环境中形成的化学条件，方法是使用一种创新的X射线散射技术，使研究人员能够在原子尺度上监测氧化铁矿物从流体中结晶的过程。除了这项研究帮助科学家设计更准确的历史(和预测)气候模型外，研究人员还将开发一个互动程序，重点关注氧化铁的地质和技术意义。这些练习将利用史密森学习中心Q？Rius和美国国家自然历史博物馆收集的大量氧化铁标本来解释这些矿物在维持国家技术和经济福利方面的重要作用。在过去的十年里，几项研究认为，土壤中赤铁矿和针铁矿的比例可以定量估计平均年降雨量，这增加了古土壤中赤铁矿/针铁矿比例作为古代气候指标的可能性。然而，赤铁矿/针铁矿替代物在奥陶系古土壤中的应用与公认的气候指标的结果相矛盾。首席调查员最近的研究表明，赤铁矿相对于针铁矿的结晶是由溶液pH、温度和时间的复杂相互作用决定的。PI的假设是，在某些条件下，针铁矿的形成与“氢化赤铁矿”的亚稳性相抗衡。氢化赤铁矿是一种在175年前发现的矿物，在20世纪20年代被证明是不可信的。具体地说，它们表明，在中低温度(130℃以下)的碱性体系(pH~8)中，氢氧化铁与氢化赤铁矿的反应是有利的。氢化赤铁矿的组成介于针铁矿和赤铁矿端元之间，从氢化赤铁矿到化学计量比赤铁矿的转变涉及结构不连续，暗示着相邻铁原子之间开始发生磁性耦合。PI将结合最先进的同步辐射X射线衍射技术和计算方法，以了解控制这一铁(氢)氧化物纳米相体系的相对热力学稳定性的因素。赤铁矿/针铁矿替代物在古气候重建中的准确应用，需要对高铁水合物转变为针铁矿和赤铁矿的速率以及影响反应产物形成的变量有严格的了解。这项提案中概述的一系列研究进展将对成熟土壤中铁(氢)氧化物的性质和演化速度提供重要的限制。因此，结果将提高赤铁矿和针铁矿作为气候指标的效用，并将揭示缺陷状态与氧化铁的物理性质和行为之间的关系。这一裁决反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Dating back to the earliest hominids, the minerals hematite and goethite (both iron oxides) have provided us with durable, non-toxic red, yellow, and brown pigments, and today these minerals are synthesized in the millions of tons. They also are of critical economic importance because they serve as the primary reserves used to extract iron, and iron accounts for 90% of the world's metal consumption. Moreover, they can control the ability of soils to remove contaminants from groundwater because they frequently form as coatings on other soil minerals, such as quartz and feldspar. Even as only a microscopic veneer, they can attract and remove toxic metals, which otherwise would not react with the quartz or feldspar hosts. In spite of their economic and environmental significance, geologists continue to debate how they form and what mechanism determines which of either hematite or goethite is likely to crystallize. Understanding this formation process matters because some scientists have used hematite and goethite ratios to infer the climatic conditions that generated ancient soils. This project strives to determine the chemical conditions that dictate whether hematite or goethite will form in natural environments by using an innovative X-ray scattering technique that allows researchers to monitor iron oxide minerals at the atomic scale as they crystallize from fluids. In addition to this research helping scientists design more accurate historical (and predictive) climate models, the Investigators will also develop an interactive program that focuses on the geological and technological significance of iron oxides. These exercises will take advantage of a Smithsonian Learning Center called Q?rius, which attracts approximately 100,000 school children each year, and the U.S. National Museum of Natural History's extensive collection of iron oxide specimens to explain the important role of these minerals for maintaining the nation's technological and economic welfare.Over the past decade, several studies have argued that the ratio of hematite to goethite in soils can yield quantitative estimates of mean annual precipitation, raising the possibility that hematite/goethite ratios in paleosols may serve as proxies for ancient climate. The application of hematite/goethite proxies to Ordovician paleosols has, however, conflicted with the results of well-established climatic indicators. Recent research by the Principal Investigators has revealed that the crystallization of hematite relative to goethite is determined by a complex interplay of solution pH, temperature, and time. The PIs hypothesize that the formation of goethite is outcompeted in certain conditions by the metastability of 'hydrohematite,' a mineral discovered 175 years ago and discredited in the 1920s. Specifically, they suggest that the reaction of ferrihydrite to hydrohematite is favored in alkaline systems (pH ~8) at low to moderate temperatures (below 130oC). The composition of hydrohematite falls halfway between those of the goethite and hematite endmembers, and the transition from hydrohematite to stoichiometric hematite involves structural discontinuities suggestive of the onset of magnetic coupling between neighboring iron atoms. The PIs will combine state-of-the-art, synchrotron X-ray diffraction techniques with computational methods to understand the factors that control the relative thermodynamic stabilities in this system of iron (hydr)oxide nanophases. Accurate applications of hematite/goethite proxies for paleoclimate reconstruction require a rigorous understanding of the rates at which ferrihydrite alters to goethite and hematite and on the variables that influence which reaction products form. The series of research thrusts outlined in this proposal will provide important constraints on the character and the rates by which iron (hydr)oxides evolve in maturing soils. The results thus will improve the utility of hematite and goethite as a climate indicator, and they will reveal the relationship between the defect states and the physical properties and behaviors of iron oxides.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.