^1,2Oleg I. Siidra, ³Artem S. Borisov, ⁴Victoria A. Ginga, ¹Veronika R. Abdulina, ⁵Dmitri O. Charkin, ⁶Anatoly V. Kasatkin, ³Astrid Holzheid, ⁴Annette Setzer, ⁷Vladimir N. Bocharov
Icarus (in Press) Link to Article [DOI: 10.1016/j.icarus.2026.117275]
¹Department of Crystallography, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
²Kola Science Center, Russian Academy of Sciences, Apatity 184200, Murmansk Region, Russia
³Institut für Geowissenschaften der Universität Kiel, Olshausenstr. 40, D-24098 Kiel, Germany
⁴Felix Bloch Institute for Solid-State Physics, Leipzig University, Linnestrasse 5, D-04103 Leipzig, Germany
⁵Chemistry Department, Moscow State University, Vorobievy Gory 1-3, Moscow 119991 Russia
⁶Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninsky Prospekt 18-2, 119071 Moscow, Russia
⁷Geomodel Resource Center, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
Copyright Elsevier

Copiapite-group minerals are among the most common hydrated iron sulfate minerals in a variety of geological environments on Earth. They are also believed to be widespread on the Martian surface. The transformation and stability of the copiapite-group minerals are examined in this study using a diverse array of methods, including low- (LT) and high-temperature (HT) single-crystal X-ray diffraction (SCXRD), LT- and HT-powder X-ray diffraction (PXRD), vacuum powder X-ray diffraction, HT-Raman spectroscopy, magnetization and heat capacity measurements. The research is conducted over a broad temperature range (−175–740 °C) and under vacuum (~ 600 Pa) conditions that are partially similar to those found on the Martian surface (from −153 °C to over 20 °C and ~ 600 Pa). The obtained results indicate that aluminocopiapite, (Al0.54Fe3+0.13)Σ0.67Fe3+4(SO4)6(OH)2(H2O)20, is unstable under low vacuum conditions and undergoes a structural transition to a post-aluminocopiapite phase, (Al0.63Fe3+0.04)Σ0.67Fe3+4(SO4)6(OH)2(H2O)12.44 with a significantly lower water and iron content and a higher aluminum content. Schwertmannite, Fe3+16O16(OH)9.6(SO4)3.2·10H2O is formed as a film/shell on the crystal surface of post-aluminocopiapite via a single crystal-to-single crystal (SC-SC) topotactic transformation and exsolution. After 14 days of exposure to air, the post-aluminocopiapite crystal with schwertmannite shell undergoes a reversible process, reverting to its initial aluminocopiapite state. A closely analogous transformation, involving partial dehydration, was observed for copiapite, Fe2+Fe3+4(SO4)6(OH)2(H2O)20, demonstrating that this behavior is a general feature of the copiapite group.
It is therefore unlikely that copiapite-group minerals would exist on the surface of Mars and in comparable extraterrestrial environments in their initial form. Instead, under Mars surface conditions with low vacuum, post-copiapites and schwertmannite may be among the most common minerals in hydrated iron sulfate mineral associations. This is relevant for decoding past geo- and climatic environments on Mars and for selecting the conditions for the return of intact samples collected by rovers to Earth.