Abstract:

Calcite is considered one of the most important minerals relevant to environment, and plays an important role in regulating water quality and controlling the migration and transformation of heavy metals. A range of calcite-strontianite “heterotype” solid solutions [(Ca1-xSrx)CO₃] (Sr-substituted calcites, Ca-substituted strontianites and their mixtures) with the bulk solid Sr/(Ca+Sr) mol ratios (X_SrCO3) of 0.00-1.00 were synthesized and their dissolution in initially-CO₂-saturated water for one year was experimentally studied at 25 ℃ . The aqueous Sr/(Ca+Sr) mol ratios (X_Sr2+,aq) were considerably higher than the solid X_SrCO3 values for dissolution of the Sr-calcites, and considerably lower than the solid X_SrCO3 values for dissolution of the Ca-strontianites. The mean values of log IAP at the steady state (≈log Ksp) were calculated to be (-8.45±0.13)-(-8.42±0.11) and (-8.79±0.04) - (-8.77±0.02) for calcite [CaCO₃] and strontianite [SrCO₃], respectively. In terms of the Sr-calcites, the log IAP values at the final constant state increased with the increasing X_SrCO3 of the solids; while decreased with the increasing X_SrCO3 of the solids in terms of the Ca-strontianites. The highest X_SrCO3 value in the calcite-type solid solutions and the lowest X_SrCO3 value in the strontianite-type solid solutions were measured to be 0.042 and 0.609, respectively. The corresponding dimensionless Guggenheim parameters were estimated to be a0=1.62 and a1=-1.46 for the calcite-strontianite“heterotype”solid solutions. In the constructed Lippmann diagrams, for the Sr-bearing calcite-type solid solutions with a low X_SrCO3, the Ca-poorer aqueous solutions were at equilibrium with the Ca-richer calcite-type solids; for the Ca-bearing strontianite-type solid solutions with a higher X_SrCO3, the Sr-poorer aqueous solutions were at equilibrium with the Sr-richer strontianite-type solids. The replacement of small amount of Sr in calcite and Ca in strontianite would decrease the stability of the solids. The results provide experimental evidence for the stability of calcite-strontianite solid solutions in aqueous solution and improve the understanding of the geochemical cycle of strontium in the environment.

Key words: Calcite, Strontianite, “Heterotype” solid solution, dissolution, Lippmann diagram