Phase-stability and magnetic properties of natural hydrogenated hematite – a potential water containing mineral on Mars

Hematite (α-Fe₂O₃) is the most prevalent iron oxide in sedimentary rocks and on the surface of Mars. Recent studies suggested that natural “hydrohematite” exists, wherein hematite accommodates H⁺ cations through compensation at the Fe³⁺-site, forming –OH bonds (i.e., structural water), with important implications for water in arid planetary environments. We present magnetic and Mössbauer characterizations to elucidate the magnetic response of H-doping on these samples, corroborating the results with first-principles calculations. The first-order reversal curves show a sharp -45° diagonal wing due to triaxial basal plane anisotropy. The Mössbauer spectra and field cooled/zero-field cooled M(T) both show the complete suppression of Morin transition (T_M) down to 5K, as opposed to T_M at ~260 K for hematite, indicating the presence of H⁺ as x-ray diffraction and electron microprobe analyses confirm the absence of the other elements. The defect calculations reveal the existence of stable bulk phase of hydrohematite, and the spin-anisotropies between the [0001] axis and the (0001) plane show an increasingly large energy difference with progressive H-doping in the structure – both interstitial and substitutional – verifying that hydrogenation of hematite would cause the T_M depression observed experimentally. These results provide a more comprehensive and fundamental understanding of natural hydrohematite and could serve as a blueprint for detecting this mineral on Mars via rover data or sample return.