Particle sizes produced by precipitation are of prime importance for subsequent process steps, and they depend on mechanisms hardly understood until now. In this paper a new approach to the understanding of particle formation by precipitation is given. The solid is formed in the microscopic "Kolmogorov vortices" where molecular mixing occurs. Loose aggregates within the vortex are cemented together by newly formed solid, the maximal size of the resulting solid aggregates being limited by the size of the vortex. Numerical calculation of the number of aggregates within the vortices and of the resulting aggregate sizes is performed, using one constant arbitrary fitting parameter. The agreement with experimental results is good, Further improvements of the model will require a better description of local fluid dynamics ("mesomixing").