Low-temperature crystallography and vibrational properties of rozenite (FeSO4·4H2O), a candidate mineral component of the polyhydrated sulfate deposits on Mars

Abstract

Rozenite (FeSO 4 •4H 2 O) is a candidate mineral component of the polyhydrated sulfate deposits on the surface and in the subsurface of Mars. In order to better understand its behavior at temperature conditions prevailing on the martian surface and aid its identification in ongoing and future Rover missions we have carried out a combined experimental and computational study of the mineral's structure and properties. We collected neutron powder diffraction data at temperatures ranging from 21 -290 K, room temperature synchrotron X-ray data and Raman spectra. Moreover, first-principles calculations of the vibrational properties of rozenite were carried out to aid the interpretation of the Raman spectrum. We found, in contrast to a recent Raman spectroscopic study, that there are no phase transitions between 21 and 290 K. We confirm the heavy atom structure reported in the literature (space group P2 1 /n) to be correct and present, for the first time, an unconstrained determination of the hydrogen atom positions by means of high-resolution neutron powder diffraction, and report the complete crystal structure at 290 K and 21 K. The anisotropy of the thermal expansion of the lattice vectors is α a : α b : α c = 1.00 : 2.19 : 1.60 at 285 K. Subsequent analysis of the thermal expansion tensor revealed highly anisotropic behavior as reflected in negative thermal expansion approximately || 〈101〉 and ratios of the tensor eigenvalues of α 1 : α 2 : α 3 = −1 : 3.74 : 5.40 at 285 K. Lastly, we demonstrated how combining Raman spectroscopy and X-ray diffraction of the same sample material sealed inside a capillary with complementary first-principles calculations yields accurate reference Raman spectra. This workflow enables the construction of a reliable Raman spectroscopic database for planetary exploration, which will be invaluable to shed light on the geological past as well as in identifying resources for the future colonization of planetary bodies throughout the solar system.