A theoretical method has been proposed for the construction of phase diagrams for silicates in contact with water. If ΔGcdenotes the free energy change due to the crystallization of a mineral from a solution of a defined composition, then it is envisaged that different mineral phases have defined regions of stability which are determined by the relative values of ΔGc's for all the phases that can exist. The phase diagram is, thus, constructed by computation of the limiting conditions for the ΔGcof each mineral species to attain the most negative value. By utilizing this method, it is possible to distinguish between stable, metastable, and unstable phases. It is concluded that all nonstoichiometric clay minerals are metastable with respect to their end-members.