The transformation of a suspension of MFI nanoslabs into colloidal Silicalite-l at 373 K was monitored with in situ low angle and wide angle X-ray scattering (XRS). The low angle region of the diffractograms taken in the course of the crystallization could be fitted by 1 to 3 Lorentzian lines, representing 1 to 3 populations of particles with different sizes corresponding to nanoslabs, intermediates, and large particles. The large particles gave rise to the Bragg diffraction characteristic of Silicalite-l zeolite. The measured X-ray data at low angles were at all times in agreement with the presence of entities that are multiples of the nanoslab, measuring 1.3 x 4 x 4 nm. The crystallization was performed in an open vessel with reflux cooler, and in a closed container. In both conditions, a consecutive conversion pattern of nanoblocks into intermediates into large particles was observed. The evolution of volume populations can be fined with first-order reaction kinetics for the conversion of the nanoslab volume into intermediates and an autocatalytic conversion of intermediates into large particles. This aggregation mechanism is supported by the interaction potentials of the different faces of the nanoslabs decorated with tetrapropylammonium cations, estimated using extended DLVO theory. The proposed mechanism can account for the nature of the intermediates, the preferential growth of Silicalite-1 in the crystallographic "c" direction, the strain in the colloidal Silicalite-l crystals in the crystallographic "a" direction, and the influence of reaction conditions on the crystallization kinetics.