What Lurks in the Martian Rocks and Soil? Investigations of Sulfates, Phosphates, and Perchlorates. Looking for jarosite on Mars: The low-temperature crystal structure of jarosite

Single-crystal diffraction of jarosite, KFe33+(SO4)2(OH)6, has been undertaken at low temperatures that proxy for martian surface conditions. Room-temperature data are consistent with literature data [a = 7.2913(5), c = 17.1744(17), and V = 790.72(11) in R3¯m], while the first low-temperature data for the mineral is presented (at 253, 213, 173, and 133 K). Data collections between 297 and 133 K show strongly anisotropic thermal expansion, with the c axis much more expandable than the a axis. Much of the anisotropy is due to strong distortion of the KO12polyhedron, which increases by 8% between 297 and 133 K. The data sets can aid in the identification of jarosite by X-ray diffraction of martian soils using the Curiosity Rover’s CheMin instrument.