Interpretation of Chemical Analyses of Clays

As a means of distinguishing between the broad groups of flay minerals, chemical analysis may be a valuable supijlement to other methods, but rarely will it remove the necessity for other methods, X-ray analysis in particular. The twoand three-layer clay minerals may be distinguished by the SiOj • AlzOs ratio or SiOz • sesquioxide ratio, but these ratios do not provide criteria for dividing either class. Among the three-layer clays, which embrace montmorillonite, nontronite, and the diverse hydrous micas, non-exchangeable K is a better criterion. I t seems probable that inontmorillonite, as found in bentonite deposits, does not contain significant amounts of non-exchangeable K, whereas all known hydrous micas contain significant, although widely variable, amounts of non-exchangeable K. The greatest value of chemical analysis inheres in the fact that by means of calculation the specific nature of the isomorphism may be determined. But the degree of confidence that can justifiably be placed in the calculated isomorphism depends on the purity of the sample analyzed. SiOs as impurity may be shown by an excess of calculated Si++++ ions, or by a deficiency of octahedral cations. I t also affects the calculated isomorphism of the tetrahedral layers of the lattice by reducing its Al content. On the other hand, AI2O3 as impurity affects the calculation in the opposite way ; that is, it reduces the number of Si**** ions per unit cell, while increasing both tetrahedral and octahedral Al*** ions. Pe as impurity has essentially the same effect as Al. In some samples it is possible to detect SiOz as impurity by means of calculation. Introduction. The two-layer clay minerals, as typified by kaolinite, can be readily distinguished from the three-layer clays such as the montmorillonites and hydrous micas, by the Si02:Al203 ratio as determined by analysis. But this ratio has only limited value: as a means of determining what specific mineral is present in a given sample, the Si02:Al203 ratio can never be relied on as a certain criterion. This is true even when the specimen analyzed is completely free from impurity. The same ratio is characteristic of all four of the different members of the kaolin group of clay minerals, kaolinite, dickite, nacrite, and halloysite. In the ease of montmorillonites from different sources, there is a rather wide variation in this ratio owing to the fact that the montmorillonites form an isomorphous series. It seems safe to say, therefore, that the Si02:Al20,'! ratio, or even the SiOo : sesquioxide ratio, or the SiOo :(Al203 ~|FciOa + MgO) ratio is not a dependable criterion of clay-mineral species. TfiOmorphisvi in Montmorillonite. In the montmorillonite group of clay minerals, chemical analysis may be used to determine the essential nature of isomorphism and in consequence to show the origin and location of the charge on the lattice. The isomorphous character of the members of this group of minerals can probably be shown in no other way. The fact that the specific nature of the isomorphism can be calculated from the analysis is what gives the greatest value to the analysis. The evidence is that the specific pattern of isomorphism occurring in montmorillonite from different sources is rather widely variable. The isomorphism may involve (1) substitution of Al for Si in tetrahedral positions in the lattice. (2) substitution of Fe for Al in octahedral coordination, {ti) substitution of Mg for Al in octahedral positions, and (4) substitution of cations of various kinds * I'rofessor of Soil Chemistrx' I'^tneritus, Department of Soil.s, Univer.sity of California, Berkolfy, California. by other kinds of cations in interlayer positions. The variations in the substitution in (1), (2), and (3) are basically what give the greatest interest to a chemical analysis. Accuracy of Analysts. The statement is often made that the chemical analj'sis must be accurate. Unfortunately, a reliable criterion for accuracy is not now available. Recently, Schlecht (1951) and Schlecht and Stevens (1951) reported analyses of two silicates by 34 specially chosen analysts. The results showed surprising discrepancies. With a sample of granite the different chemists reported variations in Si02 of 1.69 percent ; with AI2O3 differences of 2.78 percent; with Fe203 a difference of 1.38 percent; with K2O a difference of 3.41 percent; with NaoO a difference of 1.24 percent. All these differences are expressed as percent of the sample and not as percent variation of the respective compounds. The summation of all constituents ranged from 99.58 to 100.39 which is commonly thought to indicate reasonable accuracy of analysis. Since the range of totals was distinctly less than that of the major constituents, it is certain that where a given constituent, Si02 for example, was high, one or more of the other constituents must have been low. It is probable that different chemists would obtain an equally wide variation in analyzing a given clay sample. Such variation would almost certainly make rather large differences in calculated isomorphism and consequently in the distribution of charges in the lattice of montmorillonite. For this reason alone we are not justified in making dogmatic assertations about the specific isomorphism as calculated from a single analysis. Impurities. There is still another reason for caution in the interpretation of chemical analysis of montmorillonite, namely, the sample as analyzed may not have been free from impurity. Evidence on this point is presented by Mr. O.sthaus in the following paper in this bulletin, who shows that reasonably pure montmorillonite contains only minor amounts of non-exchangeable Ca, Na, and probably little, if any, non-exchangeable K. Therefore, if the sample is found to contain significant amounts of non-exchangeable Ca, Na, or K, this should be regarded as evidence that the sample is a mixture. Exchangeahle Mg. An altogether different kind' of point may have significant influence on the calculated isomorphism in octahedral coordination: that is, a considerable part of the Mg may be present as interlayer cation.s, or exchangeable Mg (Kelley 1945). Recently Dr. Foster (1951) and still more recently Osthaus (see the following paper, herein) have fully confirmed this fact. Therefore, exchangeable Mg must be determined and due allowance for the same must be made in calculating the distribution of cations to lattice positions. Method of Calcidation. In calculating lattice positions for the constituents found by analysis, the first step is to convert the analytical data into molecular equivalents. This is done by dividing percentages of each constituent by its molecular weight. Molecular equivalents are then converted into cation equivalents.