Thermal behavior of römerite over a Mars surface relevant temperature range: single-crystal and powder X-ray crystallography and magnetic properties

A number of hydrous iron sulfate minerals have been detected on the surface of Mars under extraterrestrial conditions. Nonetheless, certain inquiries regarding the properties and phase evolution of hydrous iron sulfate minerals remain unresolved and subject to debate at present. In our research, the behavior of römerite, Fe²⁺Fe³⁺₂(SO₄)₄(H₂O)₁₄, was examined by utilizing in situ single-crystal and powder X-ray diffraction while simultaneously acquiring data upon heating. Römerite is stable under low-vacuum conditions. It exhibits a significant negative thermal expansion in the α₃₃ direction throughout the entire temperature range from −173 to 77°C and on up to decomposition. There is a cooperative interaction between the rotation of the sulfate tetrahedra in the [Fe³⁺(SO₄)₂(H₂O)₄]⁻ clusters and the features of the hydrogen-bond system that determines the thermal expansion of römerite. The structure of römerite shows that the sulfate tetrahedra are the most rigid complexes, followed by the Fe2³⁺O₂(H₂O)₄ octahedra, and the Fe1²⁺(H₂O)₆ octahedra are the most flexible. High-temperature powder X-ray diffraction, thermogravimetry and differential scanning calorimetry were used to determine the phase transformations and the eventual decomposition of römerite at higher temperatures up to 740°C. The decomposition of römerite at 60°C is followed by an amorphization, a transformation into a mikasaite-like phase at ∼275°C and a further decomposition into a hematite-like phase above 550°C, associated with the high-temperature form of magnetite, Fe₃O₄, above 575°C. The magnetic behavior of römerite reveals weak interactions between the Fe²⁺ and Fe³⁺ centers, in line with the large spatial separation between these ions.