THE CHEMISTRY OF SALINE DEPOSITS

Abstract

The seven commonest ions in sea-water, and their concentrations, are shown in the following table:— | Positive ions | Gms. per 1000 gms. water | Negative ions | Gms. per 1000 gms. water | |:-------------:| ------------------------ | ------------- | ------------------------ | | | | Sodium | 10.7 | Chlorine | 19.3 | | Magnesium | 1.29 | Sulphate | 2.69 | | Calcium | 0.41 | Bicarbonate | 0.14 | | Potassium | 0.38 | | | COMMON IONS OF SEA WATER On the evaporation of sea-water, these ions may give rise to over 30 minerals, including:— SALTS: Anhydrite (CaSO4), Halite (NaCl). HYDRATED SALTS: Gypsum (CaSO4,2HO). DOUBLE SALTS: Dolomite (CaCO3,MgCO3), Polyhalite (2CaSO4,MgSO4,K2SO4,2H2O). What particular mineral comes out at any time depends on— (1) The amounts of the component ions in solution, (2) The solubility of the mineral. In conformity with the above two simple rules, saline deposits consist of four main types, as follows:— | Order of Deposition | Nature of Deposit | Typical Minerals | |:-------------------:| ------------------------------------------------------------- | --------------------------------- | | 4 | Stassfurt salts, readily soluble but low ionic concentrations | Sylvite Polyhalite | | 3 | Halite, readily soluble but high ionic concentrations | Sodium chloride | | 2 | Calcium sulphate, slightly soluble | Anhydrite Gypsum | | 1 | Carbonates, almost insoluble | Calcium carbonate Dolomite | MAIN TYPES OF SALINE DEPOSITS The last formed deposit, the Stassfurt Salts, have been the subject of intensive study by van’t Hoff and his pupils. This work shows that while there is a parallelism between the actual sequence of minerals in saline deposits and that which would be expected from chemical knowledge, there is no exact agreement and it is now clear from this, and much other evidence, that deposits have generally suffered extensive change since deposition. It is, for example, doubtful if dolomite was ever a primary product. The …