The Economic Aspect of Geology

by C. K. Leith

Geologic Features

Common salt constitutes the mineral halite, the composition of which is sodium chloride. It is rarely found perfectly pure in nature, but is commonly mixed with other saline materials, such as gypsum and anhydrite, and occasionally with salts of potassium and magnesium. The general grade of rock-salt deposits, where not admixed with clay, is perhaps 96 to 99 per cent of sodium chloride.

The ultimate source of salt deposits is the sodium and chlorine of igneous rocks. In the weathering of these rocks the soda, being one of the more soluble materials, is leached out and carried off by ground-waters, and in the end a large part of it reaches the sea. The chlorine follows a similar course; however, the amount of chlorine in ordinary igneous rocks is so extremely small that, in order to explain the amount of chlorine present in the sea, it has been thought necessary to appeal to volcanic emanations or to some similar agency. Ocean water contains about 3.5 per cent by weight of dissolved matter, over three-fourths of which consists of the constituents of common salt. Chief among the other dissolved materials are magnesium, calcium, potassium, and SO_4 (the sulphuric acid radical).

When sea water evaporates it becomes saturated with various salts, according to the amounts of these salts present and their relative solubilities. In a general way, after 37 per cent of the water has evaporated gypsum begins to separate out, and after 93 per cent has evaporated common salt begins to be deposited. After a large part of the common salt has been precipitated, the residual liquid, called a "bittern" or "mother liquid," contains chiefly a concentration of the salts of magnesium and potassium. Still further evaporation will result in their deposition, mainly as complex salts like those found in the Stassfurt deposit (p. 113).

The actual processes of concentration and precipitation in sea water or other salt waters are much more complex than is indicated by the above simple outline. The solubility of each of the various salts present, and consequently the rate at which each will crystallize out as evaporation proceeds, depends upon the kinds and concentrations of all the other salts in the solution. Temperature, pressure, mass-action, and the crystallization of double salts are all factors which influence the nature and rate of the processes and add to their complexity. During a large part of the general process, several different salts may be crystallizing out simultaneously. It is evident that gypsum may be precipitated in some quantity, and that external conditions may then change, so that evaporation ceases or so that the waters are freshened, before any common salt is crystallized out. This fact may explain in part why gypsum beds are more widely distributed than beds of common salt. At the same time the much greater amount of sodium chloride than of calcium sulphate in sea water may explain the greater thickness of many individual salt beds.

The evaporation of salt waters, either from the ocean or from other bodies of water, is believed to have been responsible for nearly all of the important deposits of common salt. This process has been going on from Cambrian time down through all the intervening geologic ages, and can be observed to be actually operative today in various localities. The beds of salt so formed are found interstratified with shales, sandstones, and limestones, and are frequently associated with gypsum. On a broad scale, they are always lens-shaped, though they vary greatly in extent and thickness.

The necessary conditions for the formation of extensive salt beds include arid climate and bodies of water which are essentially enclosed—either as lakes, as lagoons, or as arms of the sea with restricted outlets,—where evaporation exceeds the contributions of fresh water from rivers, and where circulation from the sea is insufficient to dilute the water and keep it at the same composition as the sea water. Under such conditions the dissolved salts in the enclosed body become concentrated, and precipitation may occur. A change of conditions so that mud or sand is washed in or so that calcareous materials are deposited, followed by a recurrence of salt-precipitation, results in the interstratification of salt beds with shales, sandstones, and limestones.

For the formation of very thick beds of salt, and especially of thick beds of fairly pure composition, however, this simple explanation of conditions is insufficient. The deposits of Michigan and New York occur in beds as much as 21 feet in thickness, with a considerable number of separate beds in a section a few hundred feet thick. Beneath the potash salt deposits of Stassfurt, beds of common salt 300 to 500 feet in thickness are found, and beds even thicker are known in other localities. When we come to investigate the volume of salts deposited from a given volume of sea water, we find it to be so small that for the formation of 500 feet of salt over a given area, an equivalent area of water 25,000 feet deep would be required. It has therefore been one of the puzzling problems of geology to determine the exact physical conditions under which deposition of these beds took place.

One of the most prominent theories, the "bar" theory, suggests that deposition may have taken place in a bay separated from the sea by a bar. Sea water is supposed to have been able to flow in over the bar or through a narrow channel, so that evaporation in the bay was about balanced by inflow of sea water. Thus the salts of a very large quantity of sea water may have accumulated in a small bay. As the process went on, the salts would become progressively more concentrated, and would be precipitated in great thickness. A final complete separation of the basin from the sea, for instance by the relative elevation of the land, might result in complete desiccation, and deposition of potassium-magnesium salts such as those found at Stassfurt (p. 113).

Another suggestion to explain the thickness of some salt beds is that the salts in a very large basin of water may, as the water evaporated and the basin shrank, have been deposited in great thickness in a few small depressions of the basin.