The proposed clusters were later proved to correspond to the unit tetrakis(mIm) Zn 2+ , which are usually referred to as SBUs (Secondary Building Units) in the literature. In a later report, it was found that although the tetrakis(mIm) Zn 2+ cluster is the most stable, other Zn 2+ clusters are present at the early stages of nucleation and are likely to be involved in the nucleation process. Classical models described above assume that during growth, no change in the crystalline phase of the nuclei occurs; however, under a given condition these assumptions might not hold. One of the models that account for non-classical growth is the Ostwald Step Rule, in which crystallization of the same final unit can take place by the successive formation of intermediate phases. In this model, the phase with the lower formation energy is the first to appear, instead of the most thermodynamically favorable, and then this phase will change via, for instance, dissolution-recrystallization or internal reorganization processes, leading to the final product. This kind of mechanism was also observed for ZIF-8 formation; amorphous phases together with ZIF-8 CPPs were found when either concentration or HmIm excess increases, leading to poor-faceted yet crystalline particles (see the middle region in Figure ), where growth took place at the expense of the metastable phases during the first 60 minutes. Those metastable phases will certainly depend on the solvent employed (the role of the solvent will be discussed later on). Using as an example aqueous synthesis, the metastable phases were identified as Zn (OH) 2 , Zn (OH) (N O 3 ) (H 2 O), and Following, and for higher molar excess, a three-step nucleation mechanism for ZIF-8 which proceeds via the formation of metastable phases was reported. Here, an initial phase separation into poor-and rich-solute regions was revealed, followed by the formation of an amorphous cluster by densification, as shown in the bottom row in Figure . The following step is the formation of particles through internal reorganization of the amorphous cluster. The authors also saw the sporadic formation of small clusters (< 5 nm) before phase separation. In addition, they reported that not all rich-solute regions turn into crystalline phases; some of them were observed to redissolve. There is a strong resemblance between the above-discussed mechanisms and those typically seen in zeolites, as the Nanoslab model for example, which reinforces the similarity between these two families of porous materials. Based on the points discussed, it can be safely assumed that ZIF-8 formation does not take place by a single mechanism; there are several pathways by which the material can be formed, depending on the synthetic conditions employed. Furthermore, the final characteristics of the particles (i.e., size and morphology) also rely on the operating mechanism, especially for crystalline materials. For example, particles that have growth by monomer addition, as schematized in the first row of Figure will typically present facets, as the growth proceeds in an organized manner; on the other hand, as the mechanism drifts towards polynuclear growth, facets are lost since there is no enough time for the material to grow in the most energetically favorable configuration, being the quasispherical or even spherulitic particles more common.