Hydrated iron sulfate minerals receive considerable attention from the standpoint of environmental science, and also due to extensive studies of the mineralogy of Mars. In this paper, we report on the thermal evolution of coquimbite AlFe3+3(SO4)6(H2O)12·6H2O by single-crystal X-ray diffraction (SCXRD) in the range of −173 to 77 °C. Powder X-ray diffraction (PXRD) was performed in the temperature range of −180 to 740 °C and low vacuum of 600 Pa. Magnetic properties for coquimbite are reported in the range of −271 °C to 7 °C. It was observed that coquimbite is stable between −180 °C and +145 °C and low vacuum of 600 Pa. We observed a gradual transition from coquimbite to the amorphous phase at 150 °C, followed by a transition to mikasaite at 225 °C, and a second amorphization at 575 °C, with afterward crystallization to hematite. SCXRD shows that the behavior of coquimbite with increasing temperature can be divided into two stages, with negative and strongly anisotropic thermal expansion at Stage I (−173 to −143 °C) and only positive thermal expansion at Stage II (−133 °C to 77 °C). All the O─H···O bonds remain virtually intact during the Stage I except for Ow2─H3···O2. The negative thermal expansion observed along the c axis in the LT range is a result of the simultaneous reduction of several bond lengths and angular distortions: i) decrease of Ow2─H3···O2 hydrogen bonds oriented approximately along the c axis; ii) shrinkage of M3O3(H2O)3 octahedra evidenced by the decrease in M3─O3 and M3─Ow3 bonds. The nature of the expansion of the coquimbite structure during the Stage II is better understood in terms of the orientation of [M2M32(SO4)6(H2O)6]3− clusters along the c axis. M─O and S─O bonds are only slightly affected by the temperature rise at Stage II, whereas O─H…O angular transformations seem to be the main driving force for the expansion of the coquimbite structure along the α11 direction upon heating.

Coquimbite exhibits distinct magnetic properties compared to other iron sulfates, driven by antiferromagnetic interactions within its M3─M2─M3 trimeric clusters of Fe3+. The presence of Al3+─Fe3+ site mixing in coquimbite introduces structural disorder, partially disrupting its magnetic ordering and contributing to magnetic entropy and magnetization features, such as a 1/3 magnetization plateau.

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