Effect of iron electronic transition on the volume behavior of a monosulfide solid solution at high pressure

Abstract Monosulfide solid solutions (mss) with a metal-to-sulfur ratio of 1:1 represent key sulfide phases in the Earth’s upper mantle and are commonly associated with inclusions in diamonds and mantle rocks. We synthesized Cu-bearing Fe and Ni mss (Fe0.90Cu0.08S and Ni0.92Cu0.08S) at 5.2 GPa and 1000 °C using a multianvil apparatus and investigated their high-pressure behavior by synchrotron X-ray diffraction in diamond anvil cells. The Fe-bearing mss exhibits a distinct and continuous elastic anomaly between 3.5 and 7 GPa, with increased compressibility and a relative 6% volume excess at room pressure conditions, attributed to a high-spin to low-spin electronic transition of Fe. The spin-transition is confirmed by synchrotron Mössbauer source spectroscopy measurements. For comparison, in-situ high-pressure data have been collected also on natural samples of monoclinic pyrrhotite (Fe7S8) and hexagonal troilite (FeS). As previously observed, the former shows a continuous pressure-induced spin-transition similar to the Fe-mss sample, while the latter shows two first order phase transitions. The high-density pressure sampling performed in this study reveals a continuous anomalous increase in compressibility in both pyrrhotite and troilite (FeS II) within the pre-spin transition regime, providing new insights into the volume evolution of these two structures. In contrast, the Ni-bearing mss displays a smooth compression trend without anomalies. These findings demonstrate that spin transitions in Fe-bearing sulfides significantly influence their compressional behavior and must be accounted when interpreting the trapping pressures of sulfide inclusions in diamonds and modeling the density of sulfide-bearing assemblages in planetary interiors.