The thermal stability of sideronatrite and its decomposition products in the system Na2O–Fe2O3–SO2–H2O

The thermal stability of sideronatrite, ideally Na2Fe3+(SO4)2(OH)·3(H2O), and its decomposition products were investigated by combining thermogravimetric and differential thermal analysis, in situ high-temperature X-ray powder diffraction (HT-XRPD) and Fourier transform infrared spectroscopy (HT-FTIR). The data show that for increasing temperature there are four main dehydration/transformation steps in sideronatrite: (a) between 30 and 40 °C sideronatrite transforms into metasideronatrite after the loss of two water molecules; both XRD and FTIR suggest that this transformation occurs via minor adjustments in the building block. (b) between 120 and 300 °C metasideronatrite transforms into metasideronatrite II, a still poorly characterized phase with possible orthorhombic symmetry, consequently to the loss of an additional water molecule; X-ray diffraction data suggest that metasideronatrite disappears from the assemblage above 175 °C. (c) between 315 and 415 °C metasideronatrite II transforms into the anhydrous Na3Fe(SO4)3 compound. This step occurs via the loss of hydroxyl groups that involves the breakdown of the [Fe3+(SO4)2(OH)] ∞ 2− chains and the formation of an intermediate transient amorphous phase precursor of Na3Fe(SO4)3. (d) for T > 500 °C, the Na3Fe(SO4)3 compound is replaced by the Na-sulfate thenardite, Na2SO4, plus Fe-oxides, according to the Na3Fe3+(SO4)3 → 3/2 Na2(SO4) + 1/2 Fe2O3 + SOx reaction products. The Na–Fe sulfate disappears around 540 °C. For higher temperatures, the Na-sulfates decomposes and only hematite survives in the final product. The understanding of the thermal behavior of minerals such as sideronatrite and related sulfates is important both from an environmental point of view, due to the presence of these phases in evaporitic deposits, soils and sediments including extraterrestrial occurrences, and from the technological point of view, due to the use of these materials in many industrial applications.