Comparative thermal transformations of synthetic Fe−Mn glycerate (alkoxides)

Abstract

Synthetic iron-manganese glycerates with compositions corresponding to different molar ratios of Fe: Mn, contain large amounts of H2O (up to 22%). Heating in air at ≈270°C produces a hydrated, disordered Mn-ferrite structure (jacobsite), as shown by XRD and IR spectroscopy. At this temperature no alkoxide groups are detected. TG curves show 45.6% to ≈54% weight losses at 290°C, with a sharp loss from 270° to 290°C for all samples, attributed mostly to the Curie transition of MnFe2O4. Further heating of each sample at ≈670°C results in a well-crystallized hematite and variable amounts of bixbyite. At this stage no H2O is left. Further calcination at ≈1050°C gives qualitatively the same products as at 670°C. Colour changes occur during the heating process. In admixtures of goethite with MnCO3 or pyrolusite the main difference from the counterpart alkoxide is shown after heating at 270°C, when the Fe−Mn mineral mixture produces mostly protohematite (disordered hematite) instead of disordered jacobsite resulting from the alkoxides.