for the electrostatic potential, ϕ(r), where ρ f (r) is the fixed charge density on crystal faces, ρ i (r) is the density of mobile species with charge q i , (r) is the dielectric coefficient equal to 78.5 in solution and to static dielectric constant of the mineral. The discrete distribution of charges on mineral faces is constructed on a 2D grid using trilinear interpolation. The corresponding Coulomb contribution to the free energy, which provides an exact solution for first-order electrostatics, is calculated as