Differences on the conversion of celestite in solutions bearing monovalent ions under hydrothermal conditions

The replacement of SO42− ions by monovalent ions in mineral SrSO4 crystals was investigated under hydrothermal conditions by using aqueous solutions bearing F− and OH− ions. Experiments were conducted at various temperatures (150–250 °C) for different reaction intervals (1–96 h), with M−/SO42− molar ratios of 1, 5 and 10, where M−=F− or OH−. The celestite crystals were completely converted into SrF2 crystals, at 200 °C using a F−/SO42− molar ratio=5 for 24 h. The morphology of the converted SrF2 crystals indicated that the heteroionic conversion proceeded by a pseudomorphic replacement process, because the transformed crystals maintained their original shape and dimensions. In contrast, the SrSO4 crystals were instantaneously converted into the Sr(OH)2 phase by a bulk dissolution–recrystallization mechanism, resulting in the formation of large transparent acicular Sr(OH)2 crystals. The differences on the conversion process are mainly associated with the chemical interaction between the mineral crystal and the hydrothermal fluid. In addition, the chemical stability of the converted phase with low solubility is also essential for the heteroionic conversion to proceed by the pseudomorphic replacement process.