The chromophore fading and spectroscopy analysis of lazurite in annealing treatment.

ABSTRACT

Sulfur-containing minerals play a key role in the sulfur cycle of the Earth's crust. However, there are few experimental studies and extrapolations on the stable existence and its conditions of polysulfides in the Earth's crust. Addressed this question, a mineral with sodalite group containing S3-, lazurite, was selected to be the research subject. Lazurite is a tectosilicate mineral with an incommensurately 3D modulated (ITM) structure. In this paper, lazurite powder and bulk samples were subjected to short-time (8 h), high-temperature (800℃) annealing experiments, and subsquently conducted the tests of FTIR, RAMAN, X-ray diffraction (XRD), and TG-differential scanning calorimetry (DSC). The identity of both the initial and annealed lazurite demonstrated that the silicate framework has a structural memory at certain temperatures. Also, the results indicated that the thermal behavior of lazurite with framework expansion, cell distortion, and reversion is about 550-650 ℃, 650-750 ℃, and below 450 ℃, respectively. With the increase in temperature, the framework expanded, and the cage clusters were reversibly distorted. Meanwhile, the chromophore S3- could be oxidized, and it faded following the framework shrinkage during the cooling process. Moreover, the reversible forced equilibrium of the ITM formation had a limit temperature indicating that the annealing treatment and spectroscopy analysis of lazurite blocks and samples might be used as a reference for temperature limitation markers in geological processes.