In this paper, we reviewed recent studies on dioctahedral mica clay minerals which differ from typical muscovite in some distintive properties, such as a particle sizeand crystal chemistry.
The chemical variations of the mica minerals would be due to the fine particles and which occur and/or alter homogeneously and/or heterogeniously from various materials such as smectites, micas and solutions.
Structural features of the mica clay minerals have been discussed from the view points of the mixed layer structures of mica and smectite and of the polytype. On the other hand it is also important to elucidate some properties of the composing layer itself, such as OH behaviours, cation substitutions and tetrahedral and octahedral order-disorders in the layer.
The habit of the mineral could be grouped into three platy ones which are hexagonal like, tabular and irregular like habits. Observations of the crystal habits, surface characters and inner structure by an electron microscopy are important to clarify the relations among their crystal growth, polytypes and genesis.