Raman spectroscopic study of polysulfanes (H2Sn) in natural fluid inclusions

Abstract

Sulfur-bearing methane inclusions with up to 68 mol% H2S in metamorphic quartzite from Bastar Craton (India) exhibit common ν1 vibration modes of H2S and CH4 at 2609 cm−1 and 2919 cm−1, respectively, associated with unknown Raman bands at 2488, 2503, 2574, and 2899 cm−1. The fluid inclusions generate molecular hydrogen and surplus sulfur during irradiation with a 532 nm, 25 mW laser, thus indicating photolytic breakdown of polysulfanes – H2Sn. Quantum chemistry modeling based on the density functional theory was conducted to better understand the origin of the unusual Raman bands in the SH-stretching region. Vibrations of simple H2S, H2S2, H2S3, H2S4, H2S5, S6–8 molecules were simulated together with selected complexes of n = 1–4 polysulfanes with the crown-shaped, orthorhombic cycloocta-sulfur identified in the fluid inclusions according to intense Raman bands at 60–62, 79–80, 149–151, 218–220, 441–442, and 472–474 cm−1. Theoretical calculations confirmed that the vibrations at 2488, 2503, and 2574 cm−1 reflect symmetric and asymmetric, H-bonded and non-H-bonded H2S2…S8…H2S2, H2S2…S8 and H2S…S8…H2S complexes in the crystallized sulfur melt. A small band at 2899 cm−1 is tentatively attributed to CH3–S8– and/or –CH2–S8– bonds, which also diminished during irradiation with green laser. “Phantom” bands occasionally occurring at >2580 cm−1 were identified as artificial bands, resulting from an imprecise angle of incidence of laser beam to rotatable holographic grating. Another small 2579 ± 1 cm−1 band in Raman spectra recorded using longer acquisition times represents a combination band of host quartz, rather than the A12ν4 resonant vibration of CH4 located at the same frequency.