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

Fe 氧化物形成的结构控制：赤铁矿与针铁矿生长的晶体学分析

基本信息

批准号：
1925903
负责人：
Peter Heaney
金额：
$ 47.49万
依托单位：
Pennsylvania State Univ University Park
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2019
资助国家：
美国
起止时间：
2019-08-15 至 2024-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1925903&HistoricalAwards=false
关键词：
Structural controls Fe oxide formation

项目摘要

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.

追溯到最早的原始人类，赤铁矿和针铁矿（均为氧化铁）矿物为我们提供了耐用、无毒的红色、黄色和棕色颜料，如今这些矿物的合成量已达到数百万吨。它们还具有至关重要的经济重要性，因为它们是提取铁的主要储量，而铁占世界金属消耗量的 90%。此外，它们可以控制土壤从地下水中去除污染物的能力，因为它们经常在其他土壤矿物（例如石英和长石）上形成涂层。即使只是微观单板，它们也可以吸引和去除有毒金属，否则这些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，但地质学家仍在争论它们是如何形成的以及决定赤铁矿或针铁矿中哪一种可能结晶的机制。了解这种形成过程很重要，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。该项目致力于通过使用创新的 X 射线散射技术来确定决定赤铁矿或针铁矿在自然环境中形成的化学条件，该技术使研究人员能够在氧化铁矿物从流体中结晶时在原子尺度上进行监测。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交互式程序，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尼学习中心 Q?rius 的优势，该中心每年吸引约 100,000 名学童，以及美国国家自然历史博物馆收藏的大量氧化铁标本，以解释这些矿物对于维持国家生态系统的重要作用。技术和经济福利。在过去的十年中，一些研究认为土壤中赤铁矿与针铁矿的比率可以得出年平均降水量的定量估计，从而提高了赤铁矿/针铁矿的可能性古土壤中的比率可以作为古代气候的代表。然而，赤铁矿/针铁矿替代物在奥陶纪古土壤中的应用与既定气候指标的结果相矛盾。首席研究员最近的研究表明，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 假设，在某些条件下，针铁矿的形成会被“水赤铁矿”的亚稳定性所击败，“水赤铁矿”是一种 175 年前发现的矿物，但在 20 年代已不再可信。具体来说，他们认为，在中低温度（低于 130oC）的碱性体系（pH ~ 8）中，水铁矿与水赤铁矿的反应有利于发生。水赤铁矿的成分介于针铁矿和赤铁矿端元之间，从水赤铁矿到化学计量赤铁矿的转变涉及结构不连续性，表明相邻铁原子之间磁耦合的开始。 PI 将把最先进的同步加速器 X 射线衍射技术与计算方法相结合，以了解控制氧化铁纳米相系统中相对热力学稳定性的因素。赤铁矿/针铁矿代理在古气候重建中的准确应用需要严格了解水铁矿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。该提案中概述的一系列研究重点将为熟化土壤中铁（氢）氧化物的特征和演化速率提供重要的限制。因此，研究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。该奖项反映了 NSF 的法定使命，并被认为值得通过以下方式支持：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。