Advanced analytical differentiation of α-HgS in historical mercuric sulfide preparations

This study investigated the physicochemical distinctions between two visually indistinguishable but differently processed mercuric sulfide materials: Hingula (M1) and Rasasindoor (M2). The structural similarity of the α-HgS phase was initially confirmed by XRD in both samples. SEM and Raman spectroscopy indicated an irregular, loosely aggregated structure and structural order, respectively. Similarly, TGA-DSC analysis showed subtle differences in the higher onset of thermal degradation. However, these techniques alone were insufficient for conclusive differentiation. EDX confirmed surface sulphur enrichment in M2, while XRF analysis provided important insights. It was found that the Hg:S weight ratio in M2 is closer to the theoretical ideal, while in M1, this equilibrium is not there. In particular, the arsenic content in M1 was found to be 2.63 wt.%, which is much higher than in M2, and emerged as a critical discriminatory marker. XPS analysis confirmed the stable chemical states of Hg²⁺ and S²⁻ in both. Subsequent XRD analyses show a transition from α-HgS to β-HgS under high-temperature evaporation conditions, demonstrating a vapor-phase recombination pathway rather than a solid-state reversible transformation. This study presents specific elemental markers to eliminate ambiguity in identification between α -HgS preparations with different purity profiles, an important step towards their safe and standardized use.