Evolution in the structure of akaganeite and hematite during hydrothermal growth: anin situsynchrotron X-ray diffraction analysis

Abstract

Synchrotron X-ray diffraction was used to monitor the hydrothermal precipitation of akaganeite (β-FeOOH) and its transformation to hematite (Fe2O3) in situ. Akaganeite was the first phase to form and hematite was the final phase in our experiments with temperatures between 150 and 200 °C. Akaganeite was the only phase that formed at 100 °C. Rietveld analyses revealed that the akaganeite unit-cell volume contracted until the onset of dissolution, and subsequently expanded. This reversal at the onset of dissolution was associated with a substantial and rapid increase in occupancy of the Cl site, perhaps by OH− or Fe3+. Rietveld analyses supported the incipient formation of an OH-rich, Fe-deficient hematite phase in experiments between 150 and 200 °C. The inferred H concentrations of the first crystals were consistent with “hydrohematite.” With continued crystal growth, the Fe occupancies increased. Contraction in both a- and c-axes signaled the loss of hydroxyl groups and formation of a nearly stoichiometric hematite.

Superhydrous hematite and goethite: A potential water reservoir in the red dust of Mars?

“…The conversion from Fh is the most common pathway for Hm formation in low-T natural systems (Cornell and Schwertmann, 2003;Lagroix et al, 2016). Although prior researchers have precipitated hydrohematite as a transient intermediate reactant (Peterson et al, 2018), for the first time, we successfully synthesized hydrohematite as a final metastable product from fresh two-line Fh gel. TRXRD revealed that when Fh gels were adjusted to pH 9-12 and heated at moderately low T (80-170 °C), Fe-deficient hydrohematite initially nucleated with Fe occ ∼0.50 (Fig.…”

Capacity of Chlorate to Oxidize Ferrous Iron: Implications for Iron Oxide Formation on Mars

“…Akaganeite, schwertmannite, and jarosite can undergo transformation to hematite in both aqueous solution and through solid-state transformation. Akaganeite transforms to hematite via a dissolutionreprecipitation mechanism at ambient (~28 • C) [35] and hydrothermal (70-200 • C) [36,37,[121][122][123][124] conditions. The solution pH and the presence of other solutes are important parameters that govern akaganeite transformation [121], with goethite forming instead of hematite at pH > 12.…”

“…The solution pH and the presence of other solutes are important parameters that govern akaganeite transformation [121], with goethite forming instead of hematite at pH > 12. Akaganeite transformation to hematite in hydrothermal systems sometimes precedes with the early formation of hydrohematite, an Fe-deficient, hydroxyl (-OH)rich hematite phase [124]. Akaganeite can also transform to jarosite at lower pH conditions (<pH 4.5) in the presence of sulfate [92].…”

Hematites Precipitated in Alkaline Precursors: Comparison of Structural and Textural Properties for Methane Oxidation

“…The composition is in agreement with the previous experimental results documented on minimum concentrations of monovalent Fe(III) ions at pH 8.2, which supported maximum formation of hematite [ 32 ]. Akaganeite has also been characterized as the first phase formed at 100 °C, which changed to an OH-rich and Fe-deficient hydrohematite in situ [ 33 ].…”

“…The thermal transformation of the Fe 2 O 3 _Na-70 and Fe 2 O 3 _NH-70 precipitates was compared by DTA/TG curves ( Figure 6 ). The temperature between 150 and 200 °C is sufficient for rapid conversion of FeOOH polymorphs and may, therefore, involve goethite and akageneite dehydroxylation [ 33 ].…”