Abstract Pseudomorphism of diopside grains was effected by immersing mineral grains of 1–2 mm diameter in 1 N Ni chloride solutions at 1 kbar and 200–680°C. It appears that both infiltration and diffusion-type replacement occur when NiCl 2 solutions are used. This is seen in the profile of Ni content of the pseudomorph phase (talc or serpentine) across the pseudomorph zone. The infiltration-type composition profiles were found for zones with a zone growth rate of greater than 1 × 10 −9 cm/sec . Migration of Ni into the pseudomorph zone is apparently controlled by a thin layer of Ni talc at the solution-pseudomorph interface. Changing the Ni/Mg content of the chloride solution changes the phases which pseudomorph the diopside. I N mixed (Ni-Mg) and Mg chloride solutions produce monomineralic pseudomorph zones of talc and then serpentine in going from the diopside outward. Serpentine pseudomorphs or replaces talc. This change of phases is assumed to be caused by a lower solubility of Ni silicates compared to Mg counterparts in chloride solutions which shifts the solution Si compositions that are saturated with the silicates. As the ratio Mg/Ni increases, the solution composition favors a less siliceous phase, serpentine. The gradient of silica activity in the pseudomorph zone, between diopside and the outer solution then produces two monomineralic zones. Ni diffusion profiles in the dissolving diopside, the Ni-poor composition of the talcs and the linear talc composition-temperature relation indicate that the pseudomorph solutions do not have the same Ni-Mg composition as does the bulk, outer chloride solution. The composition of the pseudomorph phase does not reflect the chemistry of this outer solution.