The polymorphous transition that occurs during the process of metasomatic replacement was observed in the experiment on the retgersite paramorph formation after nickelhexahydrite crystals. This process runs through a solution film formed on the primary crystal surface and then penetrated into the crystal along with the replacement front. One or several secondary mineral grains have ingrown into a comparatively perfect crystal up to the complete replacement; these grains are randomly oriented with regard to the crystal. A finegrained mass replaces the imperfect crystals. An aggregate of mineral grains incorporated into the fine-grained mass is an intermediate case. Single-crystal pseudomorphs have been synthesized for the first time as products of the nonisomorphic interaction. Conditions of the nickelhexahydrite metastable equilibrium and the retgersite stable equilibrium are closely related and determine the structural features of the replacement reactions for these two compounds. A relatively low supersaturation with retgersite in the vicinity of the eutonic point resulted in the nucleation and growth of secondary single crystals. A deviation from the eutonic point gives rise to the increasing supersaturation and mass precipitation of the secondary phase.