“…The activation enthalpies obtained at low temperature decrease as pressure increases as shown in Figure 6; accordingly, the activation volume is −14.47 cm 3 /mole as indicated in Table 1. The negative pressure effect on the conductivity in siderite resembles that in dolomite and magnesite as shown in Figure 7, however, the much lower activation volume of siderite (−14.47 cm 3 /mole) compared to those of dolomite (−1.00 cm 3 /mole) and magnesite (−3.95 cm 3 /mole), is probably related to the iron of bivalence-variable metallic cation in the lattice position, unlike the unable oxidized cations in Ca-Mg carbonates, i.e., Ca, Mg. Due to the high activation energy of Ca-Mg carbonates (1.64 eV for dolomite and 1.76 eV for magnesite), the hopping conduction due to the large polarons process involving magnesium or/and calcium vacancies and a trapped hole is suggested to be the dominant mechanism in previous studies (Mibe and Ono, 2011;Ono and Mibe, 2015). For ironrich carbonate, resembling to other Fe-bearing silicate minerals, e.g., Fe-bearing olivine, garnet, pyroxene, pand erovskites (Xu et al, 2000;Romano et al, 2006;Yang and Heidelbach, 2012;Dai and Karato, 2014;Sinmyo et al, 2014), the small polaron that electron-hole hopping between ferrous (Fe 2+ ) and ferric (Fe 3+ ) ions are likely to be the predominant conduction mechanism.…”