Spectroscopic Analysis of Ca–Sr, Ba–Sr, and Ca–Ba Sulfate Solid Solutions for In Situ Observations on Mars

Calcium sulfates are widely distributed in Gale crater and Endeavour crater on Mars. However, the mole ratios of Ca ^2+ and SO _3 in some Sr–Ba-rich sulfate veins are less than 1. This indicates that there may be additional solid solution phases on Mars, including Ca–Sr, Ba–Sr, and Ca–Ba sulfates, which, on Earth, are typically found in marine environments. In this study, we synthesized a series of Ca–Sr, Ba–Sr, and Ca–Ba sulfate solid solutions in the laboratory. These synthesized solid solutions were then characterized using various techniques, including X-ray powder diffraction (XRD), Raman, mid-infrared, and visible and near-infrared. Our results show that the XRD and Raman spectra of Ca–Sr and Ba–Sr sulfates varied with their cation contents. In particular, the linear relationships ( R ^2 ≥ 0.98) between the mole ratios of cation ions and peak positions of ${{\rm{SO}}}_{4}^{2{-}}$ tetrahedron ( v _1 ) in Raman spectra were obtained. The mole content of Ca ^2+ , Sr ^2+ , and Ba ^2+ can be precisely constrained in Ca–Sr, Ba–Sr, and Ca–Ba sulfate solid solution phases through linear relationships, respectively. These results are the experimental basis for analyzing in situ detection data (e.g., SHERLOC, SuperCam, and Oxia planum) and can be used to precisely constrain the mineral phases of calcium sulfate veins on Mars.