Abstract The liquid outer core of Mercury is fundamental to sustaining its dynamo. The intrinsic magnetic field depends strongly on the metal composition and related physical properties. The composition in turn is influenced by inner core crystallization and interactions with the adjacent reduced mantle. Chemical transport between core and mantle analogs was investigated using impedance spectroscopy at 5 GPa and up to 1,973 K and electron microscopy. Results support an iron outer core that contains significant amounts of alloying agents (>∼20 at.% Si, Ni, C, S). Bulk diffusion coefficients across the silicate‐core interface are 1− ${-}$5 × ${\times} $ 10−8 m2/s at 1,973 K. Electrical resistivity of Fe‐Ni‐Si‐C/S liquids was subsequently measured at 5 GPa and up to 2,123 K to estimate the thermal conductivity of the outer core, which ranges from 17 to 31 W/m.K. These low thermal conductivity values, compared to Fe‐Si liquids, could increase the power available to the dynamo during core cooling.

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