Secondary nucleation, the formation of new crystals due to the prior presence of other growing crystals, is the primary source of new particles in most industrial crystallizers. Of the various types of secondary nucleation possible, contact nucleation, wherein a disturbance of a growing crystal surface results in nuclei, is thought to be the most prevalent.
P. Elankovan and K. A. Berglund, "Technique for Obtaining Raman Spectra of Contact Nuclei In Situ," Appl. Spectrosc. 40, 712-714 (1986)