In Eq. S3 (SI: Materials and Methods), the mentioned variables parameterize a Guinier form factor, which is used to approximate any small units that may exist within the crystals, such as "bricks-in-the-wall" or "loose" particles with radii of gyration R s . Additionally, G s is proportional to the product of the volume fraction and the volume of these smaller particles. The initial as-precipitated gypsum, before 4000 s, contains small building units with a radius of ~4 nm. We interpret these units as components of a mesocrystal, consistent with our previous work (Fig. , stage I). The dissolution of gypsum with increasing temperature induces the rapid formation of a large population of nanoparticles at 4860 s, causing the radius of gyration of the small units to increase to ~5.5 nm (Fig. , stage II). Further crystallisation of bassanite results in a partial loss of the mesostructure. The constituent building blocks are only observable in the meridian direction (R s ~4 nm), while they vanish in the equatorial direction, as indicated by G s ~0 after >5400 s. This observation aligns with a decrease in the specific surface area expressed by parameter B, assuming crystal growth occurs through the coalescence of internal mesocrystal domains . Examining the meridian and equatorial trends in Fig. , the surviving mesostructure indicates that the crystals possess domains with shorter, nano-sized features aligned parallel to the long axes (c-axes) of the crystals. Such alignment correlates with the layer structure of gypsum and the channel structure of bassanite, where these anisotropic motifs align parallel to the long axes of the respective crystals (Fig. ). In the perpendicular directions, the domains fuse into a homogenous body, cancelling the mesostructure.