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

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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.