Structural and spectroscopic study of the kieserite-type (Mg,Mn)SO4·H2O solid solution at ambient and low temperatures relevant to Mars and the icy moons of Jupiter and Saturn

Abstract

The presence and importance of sulfates in our solar system have become common knowledge among the dedicated scientific community, as well as their crucial role in governing the water budget of planets such as Mars. The formation of subsurface oceans and cryovolcanism on icy moons of Jupiter and Saturn owe their existence to melting equilibria influenced by sulfate hydrates in the deeper parts of these celestial bodies. In such a setting, it cannot be ruled out that lower-hydrated sulfates including kieserite, MgSO4·H2O, are also present, given their stability under geologic pressures relevant even down to the lower mantle of the icy satellites. With regard to the composition of the water-soluble fraction in C1 and C2 chondritic meteorites presumed to correspond to that of the rocky cores of the Jovian moons, local kieserite may contain significant amounts of Mn in addition to Ni (and also some Fe). Minor Mn contents are probable even in kieserite occurring on the surface of Mars. Substantial lattice parameter changes and spectral band shifts between kieserite and its isostructural Mn-analogue szmikite, MnSO4·H2O, are well known, nevertheless neither the existence of a potential MgMn solid solution series nor its crystal chemical and spectroscopic behavior has yet been investigated.This work provides the first evidence for such a continuous solid solution, describes its structural properties, and gives a detailed insight into the position changes of prominent bands in FTIR- and Raman spectra of synthetic samples, as the Mn/Mg ratio increases, with the measurement having been performed both at ambient and low temperatures relevant for Mars and the icy satellites. It reveals that contents of Mn do not cause any significant shift in the H2O stretching vibrational bands, yet influence the position of sulfate-related vibrations much in the same way as the incorporation of other relevant transition metal cations, such as Ni. The contrasting behavior of the ν3 and ν1 stretching vibrations of the H2O molecule in case of Mn- (no shift) and Ni-incorporation (significant shift to lower wavenumber) along with their comparable influence on the position of sulfate bands allow to deduce the chemical composition of kieserite in a ternary Mg–Mn–Ni system based on spectroscopic remote sensing or in situ data from celestial bodies. The assessment of chemical composition from vibrational spectra is facilitated by the observed Vegard-type behavior, where lattice parameters as well as the spectral band positions change along linear trends with increasing substitution of Mg, regardless if by Mn, Fe, or Ni. The observed shift of mainly the H2O stretching vibrations to lower wavenumber values as temperature decreases (in contrast to the bands related to the sulfate group) match thermal behavior observed in other binary solid solutions of sulfate monohydrate phases isostructural with kieserite.