Nucleation-limited aggregation of crystallites in fractal growth

We report in this article a crystallization behavior of NH4Cl in a gel syst em, which leads to a new type of fractal aggregate. In situ optical microsc opy and atomic force microscopy are used to study the growth process and th e aggregate morphology. Microscopically the crystallites in the aggregate b ranches have remarkable correlations with their crystallographic orientatio ns, which introduces a strong effective anisotropy to the fractal growth. M acroscopically, the fractal pattern has a scaling property different from t he fractals generated by diffusion-limited aggregation and that generated b y homogeneous nucleation-limited aggregation. It seems that the fractal pat terns do not arise exclusively in the far-from-equilibrium situations; nor do they need the anisotropy for the fractal aggregation to be very low. (C) 2000 Elsevier Science B.V. All rights reserved.