THE PROCESS OF CRYSTALIZATION

The crystallization process consists of two major events, nucleation and crystal growth. Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, when stable these clusters constitute the nuclei. However when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size called critical radius, in order to become stable nuclei. Such critical size is dictated by the environment (temperature,supersaturation, etc.). It is at the stage of nucleation that the atoms arrange in a defined andperiodic manner that defines the crystal structure. ("Crystal structure" is a special term that refers to the relative arrangement of the atoms, not the macroscopic properties of the crystal (size and shape), although those are a result of the internal crystal structure). The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical radius. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization process, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained. Once the supersaturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so as to supersaturate the solution again.

(Source: http://en.wikipedia.org/wiki/Crystallization)

In this drawing I show the time based process of molecules gathering into clusters, some of them dissolving back into the solution and some of them reaching the critical size and forming nuclei, that atract more molecules and generates the crystal growth, the proces follows the time/energy curve that is usually followed by this mineralization process.