Topotaxial replacement of celestite single crystals by strontianite aggregates: Pseudomorphisation and porosity generation

Abstract

Celestite (SrSO4) single crystals immersed in highly concentrated Na2CO3 aqueous solutions (0.5 mol/l) are progressively pseudomorphised by aggregates of strontianite (SrCO3) crystals at ambient conditions. This replacement occurs by the coupling of dissolution and crystallisation reactions, eventually resulting in the formation of strontianite pseudomorphs, which reproduce the external shape of the initial celestite crystals. Experiments conducted for different reaction times indicate that the kinetics of the replacement reaction is mainly controlled by both the dissolution of the celestite substrate and the evolution of the porosity of the strontianite replacing layer. Such a porous layer allows the replacing solution to permeate towards the replacement front. During the initial stages of pseudomorph formation, the strontianite replacing layer is highly porous and the replacement front advances rapidly. As porosity decreases after a few days of reaction, the advancement of the strontianite replacement front slows down and an almost constant rate of about 0.3 µm/h is reached. Scanning electron microscope images and X-ray diffraction data have also shown that the growth of replacing strontianite aggregates occurs with a high degree of orientation with respect to the celestite substrate, i.e. the replacement is topotaxial.