Sulfidation Reaction Mechanisms of Olivine and Diopside at Mercury's Reducing Conditions

Abstract The surface of Mercury contains elevated amounts of sulfur of up to 4 wt.%. Volcanic degassing of S from the interior of the planet could have initiated sulfidation reactions between the degassed S and surface materials in the planet's past, leading to the observed surface S enrichment. Mercury's highly reducing conditions cause nominally lithophile elements, such as Mg and Ca, to become chalcophile and to form sulfides such as oldhamite (CaS) and niningerite (MgS) when reacting with S. Reactions of silicate minerals, serving as an analog for Mercury's surface, and gaseous S under highly reducing conditions (at the graphite‐CO buffer) have been investigated before, but the underlying mechanisms and kinetics of the sulfidation reaction are not well understood. In this study, olivine and diopside were exposed to temperatures between 800 and 1,300°C and experimental durations between 1 hr and 4 weeks in the presence of reduced S gas. Results show that the reaction in both minerals is initially interface‐controlled, but mineralogical changes in the substrates cause a shift toward a transport‐controlled kinetic regime. Moreover, the results are a combination of products from the primary sulfidation reactions as well as secondary processes, such as reduction in olivine and chemical re‐equilibration in diopside. These findings highlight potential processes that formed oldhamite and niningerite as well as silicate byproducts, both in enstatite chondrites and on the surface of Mercury. This is especially relevant in the light of upcoming observations by the ESA/JAXA BepiColombo mission arriving at Mercury in late 2026.