SODIUM CARBONATES: TEMPERATURE AND pCO2 INDICATORS FOR ANCIENT AND MODERN ALKALINE SALINE LAKES

Sodium carbonate minerals that form in alkaline saline lake systems provide evidence of lake water temperatures and aqueous/atmospheric pCO₂. Modern lakes in the East African rift valley (i.e., Lakes Magadi and Bogoria, Kenya) contain trona (NaHCO₃•Na₂CO₃•2H₂O), nahcolite (NaHCO₃), and gaylussite (Na₂Ca(CO₃)₂•5H₂O) (Eugster, 1980; Renaut and Tiercelin, 1994). In this study, experimental data have been acquired that refine the pirssonite (Na₂Ca(CO₃)₂•2H₂O) and shortite (Na₂Ca₂(CO₃)₃ ) equilibrium as a function of temperature (Bradley and Eugster, 1969) and the trona/nahcolite equilibrium as a function of pCO₂ and temperature (Eugster,1966).

Experiments used synthetic pure phases for all reactions and were conducted in sealed PerFluoroAlkoxy (PFA) tubing, permeable to CO₂ but not water. X-ray diffraction was used to identify minerals before and after experiments.

Trona/nahcolite experiments were conducted in the system NaHCO₃ -Na₂CO₃-CO₂-H₂O to study the reaction NaHCO₃•Na₂CO₃•2H₂O _(trona) + CO₂ = NaHCO_{3 (nahcolite)} + H₂O at various pCO₂ (1,000—10,000 ppm) and temperatures (25—85 °C) and 1 atm total pressure. Preliminary results show that the trona/nahcolite equilibrium occurs at lower temperatures and pCO₂ than found by Eugster (1966). For example, at 55 °C trona and nahcolite are in equilibrium at 3,000 ppm rather than 13,700 ppm.

The pirssonite/shortite experiments were conducted in the system Na₂CO₃-CaCO₃-H₂O and used shortite as a “seed” reactant because it forms from the dehydration of pirssonite. The reaction Na₂Ca(CO₃)₂•2H₂O _(pirssonite) + CaCO_{3 (calcite)} = Na₂Ca₂(CO₃)_{3 (shortite)} + 2H₂O was evaluated at temperatures between 25 and 100 °C at 1 atm. The results show that pirssonite dehydrates to shortite at T > 58 °C, and at T > 48 °C with NaCl added to the system. Understanding the conditions at which these minerals form and their phase equilibria provides information on brine evolution, pCO₂, and temperatures of modern and ancient lakes.