Experimental evaporation of superficial brines from continental playa–lake systems located in Central Ebro Basin (northeast Spain)

Abstract

Solutions coming from two natural playa-lake saline systems located in Central Ebro Basin (NE Spain) have been evaporated in the laboratory, in order to obtain the precise path of chemical evolution followed until high concentration stages. The lakes belong to two chemically different neutral brines: La Playa (with Na-Cl solutions) and La Salada (Na-Mg-SO 4 type). Experimental evaporation has been carried out at 25 8C until total dryness, and samples collected along the experiment have been analysed for their major components. Application of geochemical modelling techniques allowed calculation of the saturation indexes for the main saline minerals using the PHRQPITZ program, which incorporates Pitzer's model. The mineral precipitation sequence for La Playa brines following the saturation data is: gypsum, halite, thenardite and epsomite. Brines reach saturation almost simultaneously with respect to both halite and thenardite, but halite precipitates more massively and hence the solid samples collected after total desiccation were composed mainly of halite. In the case of La Salada brines, the order of precipitation is somewhat different and is as follows: gypsum, mirabilite, thenardite and bloedite. Solid samples here consisted of bloedite and thenardite, this latter formed after subaquatic dehydration of mirabilite when brines attained a peritectic point. The evolution of saturation indexes is in good agreement with mineral determinations carried out on the solid experimental samples, and it allowed us to interpret the evaporative evolution of both La Playa and La Salada brines.

Bloedite sedimentation in a seasonally dry saline lake (Salada Mediana, Spain)

“…In the sequence of mineral formation inferred for the basin, sodium sulfates are formed before bloedite precipitation (Auqué et al, 1995;Valero-Garcés et al, 2000a), in agreement with results of evaporation experiments (Auqué et al, 1995;López and Mandado, 2007).…”

“…The lake is dry in summer but the basin floor is covered by surface brines during other parts of the year, with up to about 50 cm thickness of the brine layer (Pérez et al, 1998) composition (Pueyo Mur, 1978/1979Mingarro et al, 1981;Auqué et al, 1995;Pérez et al, 1998;Valero-Garcés et al, 2000a;López and Mandado, 2007).…”

“…Textural features show no indications for thenardite formation by dehydration of mirabilite, which typically produces fine-grained pseudomorphic aggregates (Pueyo Mur, 1980;Tursina et al, 1980;Vizcayno et al, 1995). However, different textures may be produced by the mechanism proposed by López and Mandado (2007) for thenardite formation from a mirabilite precursor in evaporating Mediana brines, involving subaqueous transformation by a dissolutionprecipitation reaction during brine evolution.…”

“…Gypsum (Pueyo Mur, 1978/1979Mingarro et al, 1981;Valero-Garcés et al, 2000a) and glauberite (Mingarro et al, 1981) have been reported as salt crust components at Mediana, whereby these minerals are considered to have formed as part of a sequence that ultimately results in bloedite-thenardite precipitation (Auqué et al, 1995;Valero-Garcés et al, 2000a;López and Mandado, 2007). These minerals are in fact part of the sediment underlying the crust, which is also documented by other analyses of the mud below the salt crust (Mingarro et al, 1981;Pérez et al, 1998;Valero-Garcés et al, 2000a).…”

“…Eugsterite has not previously been detected in the basin, and it hence has never been considered in geochemical modelling of brine evolution (Auqué et al, 1995;López and Mandado, 2007). Eugsterite is a fairly common mineral in salt efflorescences (Mees and Tursina, 2010), where it forms as a subaerial precipitate.…”