Crystallography relevant to Mars and Galilean icy moons: Crystal behavior of kieserite-type sulfates

[Marco E. Ciriotti]

Referenza:
▪ Wildner, M., Zakharov, B.A., Bogdanov, N.E., Talla, D., Boldyreva, E., Miletich, R. (2022): Crystallography relevant to Mars and Galilean icy moons: crystal behavior of kieserite-type monohydrate sulfates at extraterrestrial conditions down to 15 K. IUCrJ, 9, DOI link: https://doi.org/10.1107/S2052252521012720

Abstract:
Monohydrate sulfate kieserites ( M ²⁺ SO 4 ·H 2 O) and their solid solutions are essential constituents on the surface of Mars and most likely also on Galilean icy moons in our solar system. Phase stabilities of end-member representatives ( M ²⁺ = Mg, Fe, Co, Ni) have been examined crystallographically using single-crystal X-ray diffraction at 1 bar and temperatures down to 15 K, by means of applying open He cryojet techniques at in-house laboratory instrumentation. All four representative phases show a comparable, highly anisotropic thermal expansion behavior with a remarkable negative thermal expansion along the monoclinic b axis and a pronounced anisotropic expansion perpendicular to it. The lattice changes down to 15 K correspond to an `inverse thermal pressure' of approximately 0.7 GPa, which is far below the critical pressures of transition under hydrostatic compression ( P c ≥ 2.40 GPa). Consequently, no equivalent structural phase transition was observed for any compound, and neither dehydration nor rearrangements of the hydrogen bonding schemes have been observed. The M ²⁺ SO 4 ·H 2 O ( M ²⁺ = Mg, Fe, Co, Ni) end-member phases preserve the kieserite-type C 2/ c symmetry; hydrogen bonds and other structural details were found to vary smoothly down to the lowest experimental temperature. These findings serve as an important basis for the assignment of sulfate-related signals in remote-sensing data obtained from orbiters at celestial bodies, as well as for thermodynamic considerations and modeling of properties of kieserite-type sulfate monohydrates relevant to extraterrestrial sulfate associations at very low temperatures.