Polymorphic transition from retgersite (Rt) to nickelhexahydrite (Nh), as well as the transition Rt–>Nh is going by metasomatic replacement reaction mechanism. The difference between transitions is that for Nh -> Rt reaction it is suffice of film water, but the Rt–>Nh transition needs a lot of solution. The Nh -> Rt leads to formation of classic paramorpohs, but during Rt–>Nh transition, when the temperature high than 58 oC, two zone polycrystalic aggregate of locate automorph type formats. The outside zone consists of large (up to 1 mm) crystals of Nh, the inside zone consists of little crystals of Nh. If the temperature lays between 53 and 58 oC, it is observing the collective growth of both phases without attributes of polymorphic metasomatic replacement transition.