Amorphous As-sulfide precipitates from the shallow-water hydrothermal vents off Milos Island (Greece)

Amorphous orpiment-like As-sulfides (As₂S₃) are the most common As phases precipitating in hydrothermal systems, yet there is a lack of information regarding their solid-state characterization. Using a combination of optical, SEM-EDS, micro-Raman and XANES/EXAFS applications, we investigated yellow-orange As- and S-rich sediments occurring in the shallow-water hydrothermal system off the coast of Milos Island, Greece. The precipitates have several morphologies, but are dominantly colloidal. Intriguing "biological" morphologies also exist (e.g., cell-like (~10μm), spirals (~20μm), and rounded "cinnamon bun" shapes (~20μm)). SEM-EDS data indicated that the precipitates have an As:S ratio similar to orpiment (average=0.58, range 0.51-0.63; n=8). Micro-Raman spectra indicated that orange colored precipitates appear to be dominated by poorly crystalline and/or amorphous arsenic sulfides with micro-amounts of more crystalline orpiment and impure sulfur. The yellow sediments also contained crystalline elemental sulfur in the form S₈. Bulk As K-edge XANES spectra of the As-sulfide precipitates proved a valence of As corresponding to orpiment-type (As₂S₃) compounds (-1 to +3). EXAFS fitting results indicated that the studied material exhibits an amorphous orpiment-like structure with As ions coordinated by 3 sulfur atoms (CN=3.0). The As-S interatomic distance of the first shell is calculated at 2.279Å and the Debye-Waller factor (σ²) is 0.00427. These data suggest that the modeled structure of the studied precipitates is slightly S-deficient and ordered only in the first shell around As, resembling an orpiment-type structure, whereas higher shells are not present and must be disordered. The disorder phenomenon may be strictly produced either by the existence of occasional As-S-As bridges with As-As bonds or by the occurrence of As-O-As bridges, causing twisting of the AsS₃ pyramids in the initial orpiment structure. This distortion in the higher coordination shells of the structural sheets creates the amorphous orpiment.