Low-temperature crystallography and vibrational 2 properties of rozenite (FeSO 4 ·4H 2 O), a candidate mineral 3 component of the polyhydrated sulfate deposits on Mars

: Rozenite (FeSO 4 ·4H 2 O) is a candidate mineral component of the polyhydrated sulfate 17 deposits on the surface and in the subsurface of Mars. In order to better understand its behavior 18 at temperature conditions prevailing on the martian surface and aid its identification in ongoing 19 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 21 temperatures ranging from 21 – 290 K, room temperature synchrotron X-ray data and Raman 22 spectra. Moreover, first-principles calculations of the vibrational properties of rozenite were 23 carried out to aid the interpretation of the Raman spectrum. We found, in contrast to a recent 24 Raman spectroscopic study, that there are no phase transitions between 21 and 290 K. We 25 confirm the heavy atom structure reported in the literature (space group P 2 1 / n ) to be correct and 26 present, for the first time, an unconstrained determination of the hydrogen atom positions by 27 means of high-resolution neutron powder diffraction, and report the complete crystal structure at 28 290 K and 21 K. The anisotropy of the thermal expansion of the lattice vectors is α a : α b : α c = 29 1.00 : 2.19 : 1.60 at 285 K. Subsequent analysis of the thermal expansion tensor revealed highly 30 anisotropic behavior as reflected in negative thermal expansion approximately || 〈101〉 and ratios 31 of the tensor eigenvalues of α 1 : α 2 : α 3 = −1 : 3.74 : 5.40 at 285 K. Lastly, we demonstrated how 32 combining Raman spectroscopy and X-ray diffraction of the same sample material sealed inside 33 a capillary with complementary first-principles calculations yields accurate reference Raman 34 spectra. This workflow enables the construction of a reliable Raman spectroscopic database for 35 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. pattern of the protiated rozenite capillary sample was collected at the I11 instrument, Diamond Light Source, UK using the Multi Crystal detectors The experiment was carried out at an ambient temperature of 295 ± 0.5 K. The wavelength of 0.826574(9) Å and a zero-point error of 0.000315(2) ° was determined from a NIST SRM 640c silicon standard. Raman spectra of protiated rozenite were acquired using a B&WTek i-Raman Plus spectrometer equipped with a neodymium doped Yttrium-Aluminum-Garnet laser that was to 532 of 30 We collected data over the entire spectral range accessible to the spectrometer (i.e., 65 4200 with a resolution of smaller than 3.5 -1 (as the manufacturer at 614 on protiated rozenite powder samples both outside and inside of the very same borosilicate capillary that was used for the synchrotron diffraction analysis. The spectra were collected for 53 and 50 seconds and averaged over three acquisitions for the sample outside and inside of the capillary,