The thermal stability of ·S₂^- and ·S₃^- radicals in afghanite

Polysulfide ions play a crucial role in both the lithospheric and biospheric sulfur cycles. This study reveals the coexistence of various polysulfides (·S₂^-, ·S₃^- and S₂²⁻) alongside sulfates (SO₄²⁻) and sulfites (SO₃²⁻) in afghanite. Upon subjecting afghanite to heat treatment across a range of temperature, sulfur species within the mineral undergo interconversion, leading to alterations in colour, fluorescence, and other properties. In this paper, variations in the content of different sulfur species in afghanite before and after heat treatment were observed. The findings indicate a significant conversion of ·S₂^-, ·S₃^- and S₂²⁻ to SO₄²⁻ at 300 ℃, with some ·S₂^- and ·S₃^- regenerating at higher temperature intervals due to the oxidation of S₂²⁻. This study also discusses the generation of chlorine vacancies in afghanite resulting from the inequivalent replacement of Cl⁻ by S₂²⁻, with the vacancy concentration decreasing gradually as the temperature rises. This study offers a potential approach for using the ratio of oxidized and reduced sulfur species in afghanite to assist in determining the oxygen fugacity (f_O2) of the formation environment in the future. Furthermore, the ε-cage of afghanite has the ability to encapsulate a specific amount of ·S₂^- and ·S₃^-, suggesting its potential use as a new mineral pigment.