Stability of Pseudomorphically Converted SrCO{sub}3 and SrF{sub}2 Crystals in (CrO{sub}(4-)){sup}(2-) Hydrothermal Fluids: Effect of Porosity on the Hydrothermal Conversion

摘要

The chemical stability of synthetic SrCO{sub}3 and SrF{sub}2 porous crystals was evaluated in solutions of K{sub}2CrO{sub}4 under hydrothermal conditions. Furthermore, the effect of porosity on the pseudomorphic conversion of the original SrCO{sub}3 and SrF{sub}2 crystals was investigated. The SrCO{sub}3 crystals were slightly dissolved in the hydrothermal medium, but the conversion of SrCO{sub}3 into SrCrO{sub}4 did not proceed during the treatment. In contrast, the porous SrF{sub}2 crystals were less stable in the K{sub}2CrO{sub}4 solution, and their controlled dissolution led to the formation of a layer of aggregated SrCrO{sub}4 crystals at the SrF{sub}2 crystal surface. In general, the inner porosity in the precursor crystals allowed the penetration of the hydrothermal fluid; however, the new formed SrCrO{sub}4 layer limited the incorporation of fresh hydrothermal fluid media, which in turn limited the bulk pseudomorphic conversion of the crystals. Above 72 h of treatment the thickness of the SrCrO{sub}4 layer decreased with increasing reaction time, and some differences observed on the pore morphology were associated with the dissolution and recrystallization of the unconverted SrF{sub}2, which occurred due to the presence of hydrothermal fluid trapped inside the crystal.