Salt minerals and waters from soils in Konya [Turkey] and Kenya

This study deals with the relation between the mineralogical composition of salt assemblages and the composition of groundwaters from which these salts precipitated. A comparison was made between salts and waters sampled in the Konya Basin in Turkey and waters sampled in three different regions in Kenya.The chemical composition of waters from rivers entering the Konya Basin is different from the composition of those from rivers in Kenya. The initial composition of these rivers determines the type of minerals that will precipitate during evaporation of these river waters. The ratio of calcium to carbonate at the moment the solution becomes saturated with respect to calcite, usually the first mineral to precipitate, determines whether the final solution will become carbonate-rich or carbonate-poor. The ratio of magnesium to silica and the ratio of calcium to sulphate at the moment the solution becomes saturated with respect to sepiolite and gypsum respectively, determine the magnesium and sulphate content of the final solution. In this way six different types of concentrated brines originate and six different types of salt assemblages precipitate from these brines during evaporation.The concentrated waters and the salt assemblages in the Konya Basin belong mainly to the Na-Mg-SO _{<font size="-1">4</font>} -Cl-type. The evolution of the groundwater composition and the type of minerals which precipitated from these groundwaters can be explained by assuming successive precipitation of calcite, sepiolite and gypsum.The concentrated waters and the salt assemblages from Kenya belong mainly to the Na-CO _{<font size="-1">3</font>} -SO _{<font size="-1">4</font>} -Cl-type. The behaviour of the dissolved species and the type of salt minerals can be explained by assuming calcite and sepiolite precipitation only.The crystallographic properties of some salt minerals were determined by means of X-ray diffraction analysis and the morphological properties by Scanning Electron Microscopy. The presence of halite causes a salt crust to become dense and sealing. The porosity of a salt crust increases when bloedite or trona is present.Prediction of the sequence of salt minerals which will precipitate from a concentrated solution requires an accurate knowledge of the thermodynamic properties of concentrated electrolyte solutions. Unfortunately, these properties are still unknown for carbonate-rich solutions. Only recently a theory was developed for carbonate-poor solutions.The watersamples were evaporated in the laboratory in evaporating dishes under ambient conditions. The mineralogical composition of the precipitates was compared with the assemblages that occur in the field.The carbonate-containing salt assemblages both from the field and from the laboratory experiments were investigated by means of log PCO _{<font size="-1">2</font>} - log aH _{<font size="-1">2</font>} O diagrams. It appeared that neither the field nor the laboratory salt assemblages were in equilibrium with the CO _{<font size="-1">2</font>} - pressures of the solutions from which they precipitated. These CO _{<font size="-1">2</font>} -pressures were calculated from the analytical data.The carbonate-free salt assemblages from the Konya Basin were investigated by means of Jänecke-diagrams. For a few selected samples the theoretical mineral sequences were calculated under the assumption of equilibrium precipitation with the help of the computerprogram of Harvie & Weare (1980). It appeared that prediction of mineral assemblages in salt efflorescences is difficult even with a sound thermodynamic theory. Salt assemblages in natural salt efflorescences could best be predicted assuming precipitation under equilibrium conditions, whereas mineral assemblages in evaporating dishes in the laboratory could best be predicted assuming both metastable mineral formation and fractional crystallization. This conclusion is supported by the presence of the two new minerals, konyaite and eugsterite, which have been discovered in this study. Both are metastable minerals under ambient conditions.The oxygen and hydrogen isotopic composition of some waters from the Konya Basin in Turkey and of the watersamples from the Amboseli Basin in Kenya were determined. The isotope fractionation caused by evaporation is different in groundwater and surface water. Processes of evaporation and water-rock interaction could be distinguished.