Using a modified version of the Goldmann–Hodgkin–Katz equation relative permeabilities to Ca2 + and various monovalent alkali and organic cations were calculated. Pores were found to be fairly cation selective, with a selectivity sequence determined to be Ca2 + > Li+ > Rb+ ≈ K+ ≈ Na+ > TEA+ ≈ TBA+ > Cl−.