nm -1 (compare 3200 s vs. 5200 s in Fig. ). The isotropic character and slight shift in q of this feature indicate the formation of small particles independently of the oriented ones (the 1 st hump) already present in the crystals. This supports the notion of the de novo nucleation of bassanite. In Fig. , a cryo-TEM image captured as close as possible after the onset of the gypsum-bassanite transformation (corresponding to ~4860 -5200 s in Fig. ) depicts aggregates of polydisperse crystalline nanoparticles of bassanite (electron diffraction, inset in Fig. ), with the smallest particles measuring around 10 nm in diameter. We further quantified the evolution in the small-angle scattering patterns by independently fitting the data using Eq. S3 (SI: Materials and Methods, and SI: Fig. ) for both the meridian-and equatorial-integrated curves. The trends, summarised in Fig. , provide additional insights into the mechanism of the transformations, including the presence of mesocrystallinity. These trends capture comprehensive structural features for q < 3 nm -1 and are not specific to any particular phase. However, when correlated with the diffraction peaks (Fig. and Fig. ), we can easily attribute the observed changes to the evolution of crystalline phases. In Fig. , we present a schematic illustration of the concepts resulting from the observed trends.