Heterogeneous nucleation cloud seeding

As previously noted, most zeolite syntheses of commercial value occur in systems clouded with an amorphous gel phase due to higher product yields, admitting to the possibility of homogeneous nucleation due to solubility differences, or to heterogeneous nucleation due to the abundance of foreign surface in the mediiun. Seeding these mixtures, or agitating the solutions, could induce nucleation by any of the secondary nucleation mechanisms. However, zeolite syntheses also have been conducted successfully in dilute clear alumino-silicate media, i.e., in the absence of any amorphous gel phase [14-26]. In fact, one of the early papers by Kerr [1] reported on a technique whereby dried gel was... 

In the nucleation step, there must be sites upon which the crystals can form. This is similar to seeding the clouds to cause water to precipitate (rain). There are two sources for these nucleating particles homogeneous and heterogeneous agents. 

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