Optical second harmonic signals from clay minerals

Using a neodymium glass laser and time-synchronized pulse detection, second harmonic signals were observed from a number of clay mineral powders. The second harmonic generation (SHG) experiments provide the first physical evidence (other than diffraction patterns) for the polar nature of the kaolinite layer and its stacking sequences. Well-crystallized nacrite and dickite specimens gave signals comparable to those of quartz, but the SHG intensities from the smaller kaolinite and halloysite crystallites were noticeably weaker. Based on results from eight specimens, there appears to be a direct correlation between SHG intensity and particle size, similar to that reported previously for quartz.