The process of converting exchangeable or water-soluble potassium to that occupying the position of K+ in the micas. They are counter-ions entrapped in the ditrigonal voids in the plane of basal oxygen atoms of some phyllosilicates as a result of contraction of the interlayer space. The fixation may occur spontaneously with some minerals in aqueous suspensions or as a result of heating to remove interlayer water in others. Fixed K+ ions are exchangeable only after expansion of the interlayer space.