A mathematical model is developed to describe the effect of gravity on crystallization kinetics and particulate properties in a well-mixed zeolite crystal growth process. Particle sedimentation and convection due to gravity result in a sedimentation layer, where the settled crystals compete for a limited amount of nutrient. This model is based oil heterogeneous and secondary nucleation mechanisms. It also assumes a size-dependent power-law growth mechanism that indicates a surface reaction-controlled growth rate at high supersaturation and a diffusion-con trolled growth rate at low supersaturation. The growth rate significantly depends on the number of species that participate and govern the surface integration kinetics. The predicted transient nulceation and crystallization profiles, as well as the final particle size distribution are compared with zeolite particle size distribution (PSD) data obtained from synthesis done in low earth orbit. This simulation implies that secondary nucleation dominates the crystal formation on ground but is ruled out in low earth orbit compared to heterogeneous nucleation. (c) 2005 Elsevier B.V. All rights reserved.