Determinación de la secuencia de precipitación salina en la laguna de La Playa (Zaragoza, España). 1. Condiciones de equilibrio mineral y simulación teórica del proceso

Abstract

Neutral brines of the Na-Cl type from La Playa saline system reach extremely high concentrations (up to ionic strengths of 10 molal) during the evaporative process. Ionic activities and mineral saturation indices in waters sampled along their evaporative path are calculated by PHRQPITZ code to study their geochemical evolution.
Saturation states calculated by the code have pointed out that al! brines sampled were saturated with respect to calcite, gypsum and anhydrite. However, their stability diagram shows that water activity values determine gypsum to be the more stable phase in the Ca-S0₄-H₂0 system for almost all samples. Halite, thenardite and bloedite reach saturation at a late stage during brine evolution. The evolving paths of mirabilite, glauberite, polyhalite and epsomite do not show any evidence of brine-mineral equilibrium process along evaporative evolution. Nevertheless, mirabilite saturation state should be taken with caution because of the lack of field temperature data. Calculations carried out at 15° C shows that mirabilite is affected by reequilibrium processes which take place at short temporal scales (daily fluctuations of temperature). The comparison of these results with those of Garcés et al. (1991, 1992) indicates that Pitzer' s formulation is the more adequate approach for the physicochemical treatment of highly concentrated waters.
Theoretical simulation of evaporation process has been carried out using the most diluted sample like starting point. Open system in relation to gaseous exchange between brine and atmosphere, and constant temperature (25° C) have been considered like boundary conditions along the calculations. Simulation results have faithfully reproduced the evolving path defined by means of physicochemical treatment of brine samples. This supports the previously posed hypothesis in relation to the sequence of mineral precipitation that should take place in the saline system. Moreover, this fact suggests a primary precipitation process for thenardite and bloedite, two minerals whose genesis has been commonly related with either saline efflorescence or transformation processes of previously precipitated minerals.