Clay Minerals of the Montmorillonite Group: Their Mineral and Chemical Relationships and the Factors Controlling Base Exchange

T clay minerals are of interest to various users of clays and to several groups of investigators, but to none, more so, than to those concerned in soil problems. Clays had long been looked upon as rather hopeless materials, but some years ago it became evident that a thorough knowledge of clay mineralogy was essential for the work of all groups, and the development of new methods of mineralogical research opened the way for studies of clays and related minerals. This has led to the intensive investigation of clay minerals in the laboratories of the U. S. Geological Survey and the U. S. Dept, of Agriculture, and resulted in a cooperative study of the minerals of the montmorillonite group by the authors. A study of this type must include a thorough understanding of the distinct minerals that go to make up soils, their chemical composition, the range of variations within complex mineral groups, the difference in physical properties, the chemistry and limits of base exchange, and methods of identification. However, mineralogy is not an end in itself but leads the way for studies of the conditions under which the various clay minerals form; the effects of parent rock and chemical ^and climatic conditions' that control their development. Many individuals and groups of workers in this and foreign countries have contributed to an understanding of the minerals of the montmorillonite group, and the work has seemingly progressed till a satisfactory picture of mineral relations within the montmorillonite group and the factors of base exchange can be presented. For some years it has been known that the montmorillonite group included besides aluminous members (montmorillonite and beidellite) others characterized by ferric iron, with complete isomorphism between the two. Saponite, high in magnesia, has been correlated with the group and it has been shown that lesser amounts of magnesia were characteristic of montmorillonite. A very wide variation in the ratio of Al to Si had also been demonstrated. Calcium, more rarely sodium, and smaller amounts of other bases are present and are normally exchangeable. It had been concluded that montmorillonite was made up of micaceous plates of molecular thickness with a film of interlayer water between each plate, and this has been confirmed by X-ray diffraction studies (6). Water is present in at least two forms, "High temperature water" (or more properly OH) is present to the extent of 5 to 6% and forms an essential part of the crystal lattice. The other form of water, interlayer water, is variable in amount and is closely related to the exchangeable bases. The final problem has been to determine the definite role of the various elements, to fix the limits of variation, and to correlate this with crystal structure. The study is based on the interpretation of about 100 chemical analyses, most of them new and all made on carefully selected materials. Minerals separated from soils are usually mixtures and so, in general, are unsuitable for use in mineralogical interpretation, which is the problem in the present study. Also, they commonly show marked complexity of composition and do not approach end members of the montmorillonite group but are intermediate in composition. However, there are adequate analytical data to show the general relationships of soil clays within the montmorillonite group, although much detailed work remains to be done on soil clays. Interpretation of chemical analyses presented here has as its fundamental basis the crystal structure and ionic substitutions that have been demonstrated by X-ray methods. The work of Bragg (2), Maugin (7), and Pauling (8) laid the foundation for an understanding of the minerals characterized by platy structures. Pauling considered these minerals to be made up of layer lattices, the silicate being formed by a combination of gibbsite or brucite layers with silicate layers, the atomic arrangements of which are determined by coordination of large anions about small cations. The work of Bragg and his associates showed that only Al ions could proxy Si in positions surrounded tetrahedrally by oxygen ions in the crystal lattice. Also, that a number of different cations, including Al, Fe, Fe, Mg, Li, Cr, Mn and Mn could play similar roles in the crystal structure by occupyingpositions having octahedral