Hematite-goethite ratios at pH 2–13 and 25–170 °C: A time-resolved synchrotron X-ray diffraction study

Iron (oxyhydr)oxides are sensitive indicators of pH, Eh, temperature, microbial activity, and climate conditions in the Critical Zone. The most ubiquitous environmentally significant iron oxides soils two-line ferrihydrite, goethite, hematite. Here we present a comprehensive study transformation ferrihydrite to hematite goethite over wide range temperature (25–170 °C) initial pH (2−13) through ex situ batch synchrotron X-ray diffraction (XRD) experiments. Within high time resolution our experiments, nucleated nearly simultaneously from equal concentrations by mass. Hematite increased abundance relative until steady-state ratio was achieved, both phases ceased growth on depletion ferrihydrite. Higher temperatures lower water activities favored formation at all values studied. In time-resolved, angle-dispersive an 3 5. contrast, preferentially formed neutral (initial 7–8) highly alkaline ≥ 11). Surprisingly, mildly 9–11) induced precipitation Fe-deficient (Feocc = ~0.80–0.90) variety known as “hydrohematite” greater than goethite. Our results useful for application hematite-goethite ratios paleoclimate proxies soil sediment systems with low buffering capacities.