Response to Concept 1, One of Many Possible Responses

The process of solute concentration by evaporation is a function of mineral precipitation that removes the solutes associated with the precipitating mineral. That is, as water evaporates the solutes become more concentrated until thermodynamic saturation occurs and minerals begin to precipitate. Consider for example a closed system (no escape of solutes) undergoing evaporation in which the precipitation of calcite (CaCO3) proceeds until either all the calcium (Ca+2) or all the carbonate (CO32) is removed from the solution. As evaporation continues, the next insoluble mineral that uses the remaining Ca+2 or CO32 in its structure will precipitate. For example, precipitation of anhydrite (CaSO4) will use any remaining Ca+2 as long as there is sufficient SO42 in solution. This process continues until the next insoluble mineral starts to precipitate and so on until the thermodynamic activity of the water is the same as the atmosphere and evaporation can no longer occur. In a completely closed system this precipitation process is conceptually analogous to the evolution of minerals in a cooling magma (but with different thermodynamic controls, heat for cooling magma versus thermodynamic activity of each solute for the brine system). The end point of the concentration by evaporation process is the equalization of the thermodynamic activity coefficient of the air with that of the evaporating solution, which is analogous to a cooling magma reaching the point where at least one of the constituents required to form the most thermodynamically favorable mineral has been depleted and the next mineral in the sequence begins to form from the cooling magma.

In an open system the hydrogeology is a major factor in the evolution of brine. The evolutional sequence in a closed system is a function only of the initial concentration and ratio of the individual solutes in the water entering the system. In a chemically open or “leaky” system, the solute evolution depends largely on the ratio of mass-flux discharge to mass-flux recharge. If the system is open or “leaky”, some of the solutes continually escape from the system and thus, are not available for mineral precipitation. This leakage persistently alters the solute/mineral precipitation evolution sequence. In this way the same input water may evolve to a NaCl brine, a NaHCO3 brine, a CaCl2 brine, MgCl2 brine, or another type of brine depending on the leakage ratio. The smaller the leakage ratio the more the solutes will evolve like those in a closed basin.

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A Conceptual Overview of Surface and Near Surface Brines and Evaporite Minerals Copyright © 2021 by Warren W. Wood. All Rights Reserved.