Phase Transition and EOS of Marmatite (Zn_0.76Fe_0.23S) up to 623K and 17GPa

In situ energy dispersive x-ray diffraction for natural marmatite (Zn_0.76Fe_0.23S) is performed up to 17.7GPa and 623K. It is fitted by the Birch--Murnaghan equation of state (EOS) that K₀ and α₀ for marmatite are 85(3)GPa and 0.79(16)×10^-4K^-1, respectively. Fe²⁺ isomorphic replacing to Zn²⁺ in natural crystal is responsible for high bulk modulus and thermal expansivity of marmatite. Temperature derivative of bulk modulus (∂K/∂T)_P for marmatite is fitted to be -0.044(23)GPaK^-1. The unambiguous B3--B1 phase boundaries for marmatite are determined to be P_upper(GPa)=15.50-0.016 T(°C) and P_lower (GPa)=9.94--0.012T(°C) at 300--623K.