Exchange of SO42− ions with F− ions in mineral celestite under hydrothermal conditions

The exchange of SO42− ions by monovalent ions, F− and OH−, in mineral strontium sulphate (celestite) single crystals was investigated under hydrothermal conditions at low temperature (150–250 °C), for various reaction intervals between 1 and 96 h, with different molar ratios of (F− or OH−)/SO42−=1, 5, 10. The conversion of celestite crystals into SrF2 was completed at 250 °C in NaF solutions. The conversion proceeded to SrF2 without severe morphological changes, regarding the original shape and dimension of the crystals. It was found that the conversion proceeds by a pseudomorphic ion exchange process achieved by a mechanism of dissolution–precipitation.