Phase transition of sanidine (KAlSi3O8) and its effect on electrical conductivity at pressures up to 11 GPa

Abstract

Sanidine is an important crustal mineral whose high-pressure phase, liebermannite, can be carried down to the mantle transition zone by the subducting slab. Owing to the characteristic channel structure of liebermannite, which can provide a pathway for K+ along the [001] direction, it can serve as a highly conductive phase. In this study, we measured the electrical conductivity of natural sanidine at pressures of up to 11 GPa. The results show that electrical conductivity decreased during the phase transition of sanidine to a mixture of K2Si4O9 wadeite, kyanite, and coesite at 8 GPa, and further to liebermannite at 11 GPa. Accordingly, the activation enthalpy increased from 1.24 to 1.39 eV, and further to 1.47 eV during the phase transitions. Polycrystalline liebermannite with a high concentration of K+ vacancies (35%) did not exhibit highly conductive behavior when the lattice-preferred orientation was not developed. Compared with the theoretically calculated conductivity of liebermannite with a K+ vacancy of 25% along the [001] direction, the single-crystal liebermannite was highly anisotropic (given simply by the difference between logσmax and logσmin), at least higher than 4. Furthermore, the electrical conductivity of polycrystalline liebermannite was approximately one order of magnitude lower than that measured by geophysical observations (0.1 S/m) in the transition zone beneath Northeastern (NE) China and the Philippine Sea, and could not be used to interpret high-conductivity anomalies without a preferred orientation along the [001] direction.