Stability and direct conversion of mineral barite crystals in carbonated hydrothermal fluids

Abstract

The pseudomorphic replacement of mineral barite (BaSO₄) crystals into barium carbonate was investigated in the present work by using carbonated alkaline hydrothermal fluids. Hydrothermal treatments were carried out over the temperature range from 150 up to 250 °C for intervals between 1 and 192 h, with different filling ratios (40–70%), and ${{\text{CO}}^{{2 - }}_{3} } \mathord{\left/ {\vphantom {{{\text{CO}}^{{2 - }}_{3} } {{\text{SO}}^{{2 - }}_{4} }}} \right. \kern-\nulldelimiterspace} {{\text{SO}}^{{2 - }}_{4} }$ molar ratios of 1, 5, and 10. The reaction products were characterized by XRD and SEM techniques. The chemical reactivity of mineral barite crystals was markedly limited at temperatures below 200 °C, and only a tiny BaCO₃ layer on the surface of the original BaSO₄ crystal was formed on the crystal treated for 192 h. The rate of the pseudomorphic conversion of BaSO₄ into BaCO₃, was accelerated by increasing the reaction temperature and the molar ratio ${{\text{CO}}^{{2 - }}_{3} } \mathord{\left/ {\vphantom {{{\text{CO}}^{{2 - }}_{3} } {{\text{SO}}^{{2 - }}_{4} }}} \right. \kern-\nulldelimiterspace} {{\text{SO}}^{{2 - }}_{4} }$. Powder X-ray diffraction results showed that under hydrothermal conditions the replacement of ${\text{SO}}^{{2 - }}_{4}$ ions by ${\text{CO}}^{{2 - }}_{3}$ ions, in barite crystals was completed at 250 °C with a molar ratio ${{\text{CO}}^{{2 - }}_{3} } \mathord{\left/ {\vphantom {{{\text{CO}}^{{2 - }}_{3} } {{\text{SO}}^{{2 - }}_{4} }}} \right. \kern-\nulldelimiterspace} {{\text{SO}}^{{2 - }}_{4} }$ = 10 for an interval of 192 h, resulting in the Witherite structure. The morphology of the completely converted BaCO₃ at 250 °C in a Na₂CO₃ solution for 192 h, showed that the conversion proceed without severe changes of the original shape and dimension of the original crystal, similar to that observed in mineral pseudomorphic replacement process.