At the end of 20th century, we have to summarize a progress of clay crystallography in 20th century and mention what should be needed to make a breakthrough to clay application for environmental material science in new century. Clay crystallography in 20th century illustrated that clay is a crystalline material with some stacking disorder one-dimensionally. For our 21st century with continuous development, however, we have to understand the clay surface structure and charge distribution on the surface. If we understand those, it would be available to control the performance of clay materials with atomic scale.
