An informal overview is given of the development of the fundamental particle model, where, in a series of papers published from 1982-1987, researchers from the Macaulay Institute presented a radical model for the interpretation of the crystal structure, chemistry, and genesis of interstratified clays. The model reconciled electron microscopy and X-ray diffraction (XRD) data from a variety of sedimentary clay specimens, and proposed that these minerals were composed of nano-crystalline "fundamental particles" whose adsorptive interfaces were responsible for the expanding smectite layers observed by XRD. The term "interparticle diffraction" was used to describe this phenomenon. Experiments were reported which proved the model, by producing randomly interstratified illite-smectites from combinations of ordered illite-smectite and smectite clay dispersions. The model was extended to propose that fundamental particles were the primary units of crystallization, and that changes in the particle population were responsible for the commonly observed evolution in XRD character of these minerals with increasing depth of burial and temperature.