Hydrolysis of Certain Soil Minerals

It is generally conceded that hydrolysis is the chief chemical process by which minerals are decomposed into soil, and that neither oxidation nor .carbonation will take place appreciably in the absence of water. Byers (5) has developed a mechanism of kaolinization of the feldspars based solely upon the hydrolytic action of water. Breazeale and Magistad (2) have shown that orthoclase hydrolyzes to a limited extent in water, and that one of the ultimate products of hydrolysis is in solution as potassium aluminate. Alexander and Byers (l) have shown that the chief product in the hydrolysis of anorthite is halloysitic acid. Stevens (10) finds that when various alumino-silicates are ground under water, they hydrolyze to give characteristic pH values, which correlate closely with the weathering tendency of the respective minerals. In recent years the Arizona Experiment Station has devoted considerable time to a studjof the hydrolytic alkalinity of arid soils, most of which are highly calcareous. McGeorge (7) has shown that the pH of suspensions of such soils depends in large measure upon the fineness of subdivision, the weight ratio of soil to water, and the presence of neutral salts. Buehrer and Williams (4) in an extensive study of the hydrolytic behavior of calcium carbonate in calcareous soils, have shown that this compound exhibits much the same behavior as soils, as far as hydrolysis is concerned, but in addition found that the pH is markedly affected by the presence of colloidal substances and the chemical and physical nature of the carbonate itself. All of the foregoing evidences indicate that the hydrolytic process by which the high pH of arid soils is brought about, is quite complex and it seems desirable to learn something of the hydrolytic behavior of the two principal mineral classes represented in the soil, namely, the carbonates and alumino-silicates. Thepublished data in this field are very meager, and in many instances were not obtained under identical conditions, so that interpretations are difficult. In view of the fact that the weight ratio of mineral to water so seriously affects the pH of these suspensions, the very carefully determined pH data of Stevens (loc. cit.) are not in all cases reproducible, since the ratio was not definite. The ultimate aim of this investigation is to obtain numerical values of the dissociation constants of the acids corresponding to the various minerals and to correlate these constants with other properties of the minerals. Measurements accordingly have been made on important representatives of these two mineral classes, the samples being in all cases ground to 100 mesh, and brought to equilibrium with carbon-dioxide-free distilled water. In the case of the carbonates, precipitates of calcium and magnesium carbonate prepared under carefully controlled conditions were also studied for comparison. The natural mineral specimens were obtained from Ward's Natural Science Establishment, Inc., Rochester, New York, and from R. M. Wilke, Palo Alto, California, and were declared by the dealers to be the most characteristic forms available. They were carefully selected, pulverized, and screened. They were not treated chemically in any way, (except the bentonite which was saturated with calcium), so that their normal behavior in nature might be more closely approximated. The dilutions covered the range, 1:2, 1:5, 1:1G> 1:25, 1:50, and 1:100. After an hour of shaking on a reciprocating machine, the samples were allowed to stand a sufficient length of time, usually several days, for the hydrolysis to come to equilibrium. All