Crystallography relevant to Mars and Galilean icy moons: crystal behavior of kieserite-type monohydrate sulfates at extraterrestrial conditions down to 15 K

Abstract

Monohydrate sulfate kieserites (M 2+SO4·H2O) 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 (Pc ≥ 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+SO4·H2O (M 2+ = Mg, Fe, Co, Ni) end-member phases preserve the kieserite-type C2/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.

Incommensurate to normal phase transition in malayaite

“…In the low-temperature regime below 150 K and above the i-n transition, thermal expansion normal to [001] is absent and various phonon modes soften on cooling, while above 150 K malayaite exhibits conventional thermal expansion with all Raman active phonons except modes 1 and 3 softening on heating. Negative thermal expansion as seen along [010] of malayaite between 50 K and 180 K (Figure 6) has also been observed in several kieserite-type compounds (Wildner et al 2022), where it extends over a wider temperature range. In order to check whether the normal crystal structure is affected by the anomalies seen in the lowtemperature regime, third-order anharmonic displacement parameters for the heavier atoms Ca and Si were tentatively included in the refinement.…”

“…This could help to explain the anomalous thermal expansion normal to [001], as the anharmonicity of the potential is going to shift the average position of the SiO4 tetrahedron towards the average Ca position for larger mode amplitudes, corresponding to higher temperatures, effectively causing the observed negative thermal expansion along [010]. A very similar contraction on heating has been observed for kieserite (Wildner et al 2022), where the Ca atom is completely absent and the corner oxygen atoms of the SiO4-tetrahedron are involved in hydrogen bonding instead. Attempts to refine similarly anharmonic temperature displacements using the synchrotron data collected for crystal 1 below 90 K were unsuccessful, with Si becoming non-positive-definite in this case.…”

Synthesis and crystal structure of Fe[SeO4]OH and prediction of polytypes in the extended R[MO4]Z family

“…S2† illustrates these structure types. Thus, the 1M polytype can be considered as a characteristic of many compounds belonging to kieserites, 9–13 and to Al[SO 4 ]OH, 20 Fe[SO 4 ]F, 21,22 Sb[PO 4 ]O (ref. 23) and CaCrF 5 (ref.…”

“…S2 † illustrates these structure types. Thus, the 1M polytype can be considered as a characteristic of many compounds belonging to kieserites, [9][10][11][12][13] and to Al[SO 4 ]OH, 20 Fe[SO 4 ]F, 21,22 28 and U[PO4]F, 29 which belong to three structure types (Table 3, row 1O; Table S2 †). The predicted partially ordered 2aO polytype was reported for Ge[PO 4 ]OH.…”

“…8) belongs to the kieserite type, 9,10 which is well known for many related synthetic compounds and many minerals 11,12 including some rocks collected on Mars. 13 It crystallises in the monoclinic space group C 2/ c with unit cell parameters a ≈ c ≈ 7.5 ± 0.3 Å, b ≈ 7.2 ± 0.2 Å, and β ≈ 120.0°. Natural Fe III [SO 4 ]OH also crystallizes in the kieserite-like monoclinic modification, 14 but being synthesised by hydrothermal treatment, 15 it exhibits the β-V[SO 4 ]O structure type which is also known as pauflerite.…”