Thermal expansion of oldhamite, CaS: Implication for the surface of Mercury

The temperature excursion variation of Mercury's surface may significantly change crystal structure of surface-comprising minerals. The thermal stability of oldhamite (CaS) was investigated to validate its presence on the Mercury's surface. In particular, X-Ray Powder Diffraction (XRPD) and Thermogravimetric Analyses (TGA) on synthetic powder calcium sulfide (Alfa Aesar) were performed with the aim of confirming its stability up to 723.15 K, the highest temperature that is recorded for the surface of Mercury. Our results by XRPD and TGA results confirmed that CaS phase is stable within the daily temperature excursion on Mercury surface. Thermal expansion analyses determined the thermal expansion volume coefficient of αV = 4.03 × 10−5 K−1. The results of this work support the presence of Ca-sulfide phases on Mercury's surface and provide valid tools for interpreting the data that will be collected by the BepiColombo space mission (European Space Agency and Japanese Aerospace Exploration Agency) to Mercury.