Mutual Transformations of Polysulfide Chromophore Species in Sodalite-Group Minerals: A DFT Study on S₆ Decomposition

It is known that various polysulfide species determine the color of sodalite-group minerals (haüyne, lazurite, and slyudyankaite), and that heating induces their transformations and color change, but the mechanisms of the transitions are unknown. A prominent example is the decay of cyclic S₆ molecule. Using density-functional simulations, we explore its main decay pathways into the most probable final reaction products (the pairs of radical anions S₃⋅^-+S₃⋅^- and S₂⋅^-+S₄⋅^-). It was found that the most favorable reaction path involves initial capture of one electron by the S₆ molecule, which greatly facilitates its decay of S₆ and leads to the opening of the S₆ cycle, and subsequent decomposition of the thus formed chain radical anion, with a limiting energy barrier of ~0.4 eV. Neutral polysulfide molecules capture one electron with a significant energy reduction. The radical anions S_n⋅^- (n=2-6) are the most stable ones among corresponding species with the same n values and different charges. The capture of the second electron by S₆⋅^- occurs with a huge energy barrier (~2 eV). The results of the DFT calculations are in agreement with experimental data on the products of thermal conversions of extra-framework S-bearing groups in sodalite-group minerals.