Mineralogical Problems of Intermediate Clay Minerals

Abstract On the basis of mineralogical studies on many kinds of clay minerals found in Japan, the writers attach great importance to subtle variations in clay minerals and discuss them in the light of the concept of intermediate minerals. An intermediate mineral behaves partly as one mineral (say A) with certain treatments, and as mineral B under other conditions. There are two types of intermediate minerals: the deviation type and the mixed-layer type. The deviation type is homogeneous enough in its structure that we can describe it as composed of a combination of clearly different layer groups. The mixed-layer type has an interstratified structure of two or more kinds of layers. Furthermore it is suggested that each component layer of the mixed-layer type, in general, shows the properties of the deviation type. As an example of a mineral that can be discussed in light of the intermediate mineral concept, the writers describe the properties of very complex mixed-layer minerals related to mica clay minerals associated with epithermal ore deposits in Japan. Normally the mixed-layer minerals are found where there has been successive attack under different conditions of chemical environment or in an area that is transitional between two different chemical environments. Intermediate minerals in general are considered to be a mineral state (mineral configuration) modified in various degrees from an original mineral in response to successive changes of environmental conditions. The above viewpoints are supported by study of many clay specimens collected from alteration zones of epithermal ore deposits in Japan. It is considered that formation of intermediate minerals may be originated because of a latent defect structure in the original mineral. Polar character that is due to unequal distribution or ratio of the tetrahedral cations in a silicate layer may be favorable for the formation of the regular mixed-layer structure. Finally, intermediate minerals may be arranged according to the following scheme: Mineral A—deviation type of mineral A—mixed-layer mineral A-B—deviation type of mineral B—mineral B.