Interpretation of deep earth structures from electromagnetic data requires the constraint from the electrical conductivity of various minerals experimentally measured at high temperature and high pressure. However, the combination of these measured conductivities of different minerals always fails to match the conductivities of the multiphase rocks under in-situ conditions. To investigate the effect of ion segregation at grain boundaries on bulk conductivity, we measured the electrical conductivities of quartz, albite, and orthoclase single-phase aggregates, as well as those of two multiphase aggregates made up of the three minerals at both ambient pressure and 1 GPa over a range of temperatures. The electrical conductivities of the multiphase aggregates were an order of magnitude higher and the activation enthalpies were lower than those of the three single-phase aggregates. A significant dependence of conductivity on grain size was identified in the multiphase aggregates but not in the single-phase aggregates. The interdiffusion of alkali ions between orthoclase and albite initiated grain boundary ionic conduction, which enhanced the bulk conductivity of the multiphase aggregates to 20 S/m at 1073 K. This conduction mechanism might explain the electrical conductivity anomalies of the active shear zone in the crust.