The process of entrapment of ammonium ions in interlayer spaces of phyllosilicates, in sites similar to K+ in micas. Smectites, illites and vermiculites all can fix ammonium, but vermiculite has the greatest capacity. The fixation may occur spontaneously in aqueous suspensions, or as a result of heating to remove interlayer water. Ammonium ions in collapsed interlayer spaces are exchangeable only after expansion of the interlayer.