The Clay Minerals of California Soils

In view of the now well-established fact that inorganic soil colloids commonly contain crystalline substances, the logical next step is to determine just what specific crystalline minerals occur in the colloids of different soil types and soil series. As will be emphasized presently, the outcome of such an inquiry may shed important light both on soil processes and on geological questions. Hendricks and Fry (6) and Kelley, Dore, and Brown (8) pointed out that certain soil colloids give X-ray diffraction patterns similar to that of bentonite, which is composed largely of montmorillonite, while other soil colloids were found to resemble kaolinite or halloysite. Alien (l) and Bray (4,5) claim that beidellite predominates in certain Illinois soils, Marshall (ll) has reported beidellite in the Rothamsted soil, and certain German and Russian workers have reported montmorillonite as the dominant clay mineral in certain European and African soils. We have been able to devise a method for the study of this problem based largely on the X-ray studies of Hofmann (7) and the dehydration investigations of Ross and Kerr (14) and Kelley, Jenny, and Brown (10), by means of which the important groups of the clay minerals can be positively distinguished even when present in intimate mixtures. The method involves X-ray examination of samples at different stages of dehydration. As Hofmann showed, the montmorillonite-beidellite group of clay minerals (referred to throughout this paper simply as montmorillonite) is characterized by the fact that the position of one of its X-ray lines is variable depending on the water content of the 'sample, all other X-ray lines of the material being unaffected below about 500°C. Kaolinite and halloysite, on the other hand, show no such property. In fact the variable X-ray line of the montmorillonite group is altogether absent from the X-ray pictures of kaolinite and halloysite. However, both of these lastnamed minerals show a characteristic X-ray diffraction line, corresponding to a spacing of about 7.3 Angstrom units, which line is not given by montmorillonite. Another property highly important for X-ray identification is stability of these minerals at'about 500°C. With the exception of the variable X-ray line of montmorillonite and the 7.3 Angstrom spacing of the kaolinite group Just referred to, several of the other prominent X-ray lines of these two types of minerals practically coincide. Moreover, a number of other minerals, for example, the micas, saponite, and chlorite give X-ray spacings closely similar to some of the important spacings of montmorillonite and kaolinite. However, somewhat below 500°C. the OH ions of the lattice of kaolinite and halloysite pass off as water vapor, which destroys the crystal structure of these minerals. On the other hand, the corresponding OH of the montmorillonite group of clays is not driven off at 500°C. Therefore, X-ray analysis of samples at ordinary temperature and after heating to constant weight at 500°C., affords an additional means of distinguishing between the montmorillonitic and the kaolinitic classes of minerals. Moreover, it is possible to determine which X-ray lines are due to these clay minerals and whether still other crystalline substances are present, by heating the sample somewhat above 600°C., at which temperature the X-ray diffraction pattern of the montmorillonitic clays is also destroyed. Our results show that in addition to the clay minerals many soil colloids also contain more or less quartz and also certain other crystalline minerals that are probably related to the micas. This is in agreement with the conclusions recently drawn by AntipovKarataev and Brunowsky (2). We have investigated the inorganic colloids of different horizons of .a considerable number of soil types both from California and other states. The results show that the Yolo from California and certain horizons of a Susquehanna profile from Alabama are characterized by a clay mineral similar to the montmorillonite-beidellite group, whereas, the Cecil from Alabama and the Sierra,