Structural Behavior of Natural Silicate-Carbonate Spurrite Mineral, Ca5(SiO4)2(CO3), under High-Pressure, High-Temperature Conditions.

We report on high-pressure and high-temperature angle-dispersive synchrotron X-ray diffraction and high-pressure Raman data up to 27 GPa and 700 K for natural silicate carbonate Ca5(SiO4)2(CO3) spurrite mineral. No phase transition was found in the studied P-T range. The room-temperature bulk modulus of spurrite using Ne as the pressure-transmitting medium is B0 = 77(1) GPa with a first-pressure derivative of B0' = 5.9(2). The structure compression is highly anisotropic, the b axis being approximately 30% more compressible than the a and c axes. The volumetric thermal expansivity value around 8 GPa was estimated to be 4.1(3) × 10-5 K-1. A comparison with intimately related minerals CaCO3 calcite and aragonite and β-Ca2SiO4 larnite shows that, as the composition and structural features of spurrite suggest, its compressibility and thermal expansivity lie between those of the silicate and carbonate end members. The crystal chemistry and thermodynamic properties of spurrite are discussed.