Nucleation, growth, and phase transformation mechanisms of the iron (oxy)hydroxides

The precipitation of akaganeite and its transformation to hematite under hydrothermal conditions was monitored using a combination of in situ and ex situ experimental techniques. Using synchrotron time-resolved X-ray diffraction (TRXRD), in situ experiments between 100 and 200°C revealed that akaganeite was the first phase to form, and hematite was the final phase between 150 and 200°C.;A transient Fe-poor, OH-rich hematite phase with a chemical formula consistent with "hydrohematite" was identified in our in situ TRXRD experiments at 150, 175, and 200°C. Rietveld analyses revealed that water concentrations in the first-formed crystals of hydrohematite were comparable to water contents of FeOOH phases, such as akaganeite and goethite. Distinct peak splitting was observed in the 200°C hydrohematite diffraction patterns, indicating a violation of the 3-fold rotational symmetry of S.G. R-3c. We present a new structure refinement for hydrohematite using a monoclinic space group (I2/alpha). To our knowledge, this is the first lower symmetry hematite-like phase captured with XRD.;Kinetic analyses using phase abundances derived from Rietveld calculations of TRXRD data demonstrated that the rates of akaganeite crystallization and dissolution, hematite nucleation and crystallization increased with temperature. However, in experiments with target temperatures between 150 and 200°C, akaganeite nucleated instantaneously once experimental temperatures reached ∼123°C. Thus, the activation energy for akaganeite nucleation over the temperature range of 150 to 200°C was 0 kJ/mol. The activation energy for the crystal growth and dissolution of akaganeite was 74 +/- 8 kJ/mol and 125 +/- 7 kJ/mol, respectively. Our calculated activation energies for hematite nucleation and crystal growth was 80 +/- 13 kJ/mol and 110 +/- 21 kJ/mol, respectively.;FESEM and TEM characterization of samples heated to 150°C revealed that two akaganeite morphologies---rectangular rods and spherulites---developed simultaneously. The first hematite crystals that we detected adopted the spherulitic morphology of the precursor akaganeite. These observations suggest that hematite nucleated heterogeneously within the akaganeite spherulites in our system. We propose that the transformation from akaganeite to hematite proceeded via short-range dissolution and reprecipitation leading to pseudomorphic replacement. The akaganeite rods persisted during the first stages of pseudomorphism, and in the final stage of transformation, the growth of hematite spherulites proceeded at the expense of the akaganeite rectangular rods.