The introduction of water into the pore space of naturally occurring magnesite (leukolite) induces an intense relaxation mechanism, which is related to the electric double layer (EDL) formed on opposing sides of the solid – mater interface. The relaxation of the EDL is studied by using Broadband Dielectric Spectroscopy at different conditions of combined temperature and hydrostatic pressure. The temperature evolution of the characteristic relaxation frequency, reveals two successive temperature regions: in the low temperature one, protonic conductivity over the network of water molecules in on the solid surface, couples to the ionic transport of charged defects occurring in the solid. At higher temperatures, near – zero activation energies are found: the phenomenon is discussed theoretically and attributed to decoupling and an exchange of protons with lattice site magnesium cations, the formation of a proton enriched sub-surface layer of magnesite and subsequent release of cations to the liquid. The values of the activation volume and their independence on temperature, support the aforementioned interpretation.