Samples of nontronite and montmorillonite were digested with dilute hydrochloric acid for various periods of time. Cllemical determinations of the acid-soluble ions were made for each period of digestion. Curves are presented showing the rates of solution of these minerals ;as decreasing exponential functions of time. A change in the slopes of the curves, as plotted on semilogarithmic paper against time on the linear scale, suggests different rates of solution for the ions in the tetrahedral and octahedral layers. The initial slope of the curves represents the simultaneous rates of solution of the octahedral and tetrahedral layers with the octahedral layer predominating. The second slope of the curves represents the rate of solution of the tetrahedral layer predominantly. Extrapolation of this slope to zero time is thought to give the percent of the tetrahedrally substituted ions within very close limits. Solubility curves of nontronite suggest the presence of both iron and aluminum in the tetrahedral layer. The solubility curves for Polkville montmorillonite indicate there is no substitution for silicon in the tetrahedral layer. Experimental values of tetrahedral substitution in nontronite check very closely with values postulated from the theoretical calculations obtained from the chemical analysis of the mineral.

Several montmorillonites and a nontronite were digested in hydrochloric acid at constant temperature for various periods of time. Soluble iron, aluminum, magnesium, and in two samples titanimn, were determined for each digestion period. Rate of solution curves were obtained by plotting the logarithm of the percent of the residual ions against time. In general, the solution curves were straight lines or curves that could be resolved into two straight lines. From the interpretation of the dissolution curves, it was possible to determine the amount of the ions in octahedral and tetrahedral coordination. For some of the samples, the rate curves also gave the amount of iron and/or aluminum extraneous to the clay lattice. The amount of silicon in the clay lattice was calculated using the experimentally determined octa, hedral cations, substituted tetrahedral cations if any, and the exchangeable cations assuming 44 charges per unit cell. The distribution of cations in lattice layers indicates that the eight montmorillonites are substantially dioctahcdral. Two of the samples showed the presence of iron in tetrahedral coordination while all the samples showed extraneous silicon. The presence of extraneous silicon, iron, and aluminum in purified samples indicates that formulas derived from bulk chemical analyses can be inaccurate. The dissolution of the ions from the clay lattice determined at several different temperatures in one sample and at two acid concentrations in another sample is in agreement with chemical kinetic principles and is a first-order reaction. The rate constant for the acid dissolution reaction increased proportionately with the acid concentration. The activation energies were determined for the solution of iron and aluminum in montmorillonite from Polkville, Mississippi, and nontronite from Garfield, Washington, using Arrhenius' Law. In both samples the activation energies (17 to 18 kilocalories per mole) were the same. In the nontronite sample the activation energies were identical for what have been interpreted as octahedral and tetrahedral ions.

Pal•tic•ll•^i of montiiiorillimitp i-ppreseiitiii^' a CDiisidorablc ran^c of sizes wow separated from cnide lientonites by means of sedimentation flnd superceiitrifugatioii. Except for the effect of impurities, it was found tliat l)ase-exclianf,'e capacity was independ<'nt of particle size.

A procedure is outlined for using chemically prepared silicic acid for the removal of fluorine after decomposition by hydrofluoric acid and prior to analysis of insoluble silicates.

By combining existing procedures, direct determination of sodium and potassium can be accomplished without resort to a carbonate-chloride fusion. Analytical results are reported for the National Bureau of Standards clay sample No. 98.