THE terms interlayzring, mixed-layering, and interstratification describe phyllosilicate structures in which two or more kinds of layers occur in a vertical stacking sequence, that is, along c* or a line normal to (001). Phyllosilicate layers are strongly bonded internally but rather weakly bonded to each other. Thus, each layer approximates a one-dimensional “molecule” in the c* direction, and a two-dimensional crystal in the a and b directions. The basal surfaces of different kinds of layers are geometrically very similar and consist of sheets of oxygen or hydroxyl ions in quasi-hexagonal array. Consequently, layers with different internal arrangements can stack together and still articulate well at their interfaces. These structural factors are almost unique to the clays and phyllosilicates generally, and doubtless are responsible for the common occurrence of interstratified species. The existence of interstratified clay minerals has been known for many years. Early papers include the work of Gruner (1934), Alexander, Hendricks, and Nelson (1939), and Nagelschmidt (1944). The widespread occurrence of interstratified minerals was documented by the comprehensive study of Weaver (1956). Reported instances of interstratified clays comprise a relatively small number of tlpes despite the large number of possibilities. Dominant species involve two components, although three component minerals have been reported (Weaver, 1956; Jonas and Brown, 1959; Foscolos and Kodama, 1974). It will be shown later that small amounts (~5%) of some types of layers in three-component systems can easily escape detection by routine X-ray diffraction methods. Consequently, three-component (or more?) minerals may be more common than a reading of the contemporary literature would suggest.