Aiming at tailoring optical properties, the precipitation of LaF3 nano-crystals in LaF3-Na2O-Al2O3-SiO2 glass-ceramics is studied thoroughly on the nano-scale using advanced transmission electron microscopic techniques. Nano-sized phase-separation droplets enriched in lanthanum and silicon are formed already in the base glass. Within these less than 20 nm large droplets, LaF3 crystallizes upon heat treatment. The nano-crystallization mechanism revealed is self-limited since growth is restricted by the size of the droplets. An average crystallite size of around 12 nm is achieved with a narrow size distribution since the phase-separation droplets also contain silicon not incorporated into the growing crystal. Instead, excess silicon relocated to the periphery of the pre-existing phase-separation droplets forms a diffusion barrier around the LaF3 nano-crystals preventing further crystal growth and/or ripening. © 2009 Elsevier B.V. All rights reserved.