Electrical conductivity of siderite and the effect of the spin transition of iron

Abstract

We have conducted electrical conductivity measurements of FeCO₃ siderite under high pressure up to 63 GPa in order to understand the nature and effect of iron spin transition and its influence on the geophysical properties of siderite, which is an end-member of major carbonate minerals. The results from Raman and Mössbauer spectroscopic measurements show that the high- to low-spin transition of iron occurs at around 50 GPa in agreement with previous studies. A sharp decrease of the electrical conductivity was also observed at around 50 GP, which is associated with the spin transition in iron. Although the stability of FeCO₃ siderite may be limited under high-temperature conditions along with the mantle geotherm, solid solutions in the MgCO₃-FeCO₃ system, Mg_1-xFe_xCO₃, could be stable up to the pressure-temperature condition of the lowermost mantle. The pressure-temperature range of the spin transition in Mg_1-xFe_xCO₃ is narrower than those of the major lower mantle minerals, ferropericlase and bridgmanite, and thus the drop of the electrical conductivity induced by the spin transition could be clearer under lower mantle conditions. Therefore, the existence of Mg_1-xFe_xCO₃ may affect the observed heterogeneity of electrical conductivity in the mid-lower mantle.