The morphological and size-distribution changes of the crystals during the gypsum-bassanite phase transformation (Fig. , Fig. ) are also evident at low-angles (SI: Fig. , Fig. ). In Fig. , we present SAXS data from three characteristic moments: 3200 s when only well-developed gypsum is present at T = 35 °C; 5200 s, which marks the crossover between the dissolution of gypsum and the formation of bassanite; and 7200 s when bassanite exists at T = 90 °C. Our minimum q was limited to 0.1 nm -1 , which corresponds to a maximum feature size of ~60 nm. However, gypsum and bassanite crystals typically grow to many microns in size and beyond , and therefore, we observe them in scattering predominantly as Porod interfaces following approximately an I(q) ∝ q -4 dependence (dashed line in Fig. ). Such an interface is observed in all the curves for at least q < ~0.3 nm -1 , indicating that larger crystals are always present. A comparison of scattering features from 3200 s (53.3 min) and 7200 s, i.e., pure gypsum vs. bassanite, reveals that the initially precipitated gypsum crystals from the first cycle (Fig. ) exhibit a deviation from a Porod interface at q ~0.7 nm -1 (i.e., the 1st small "hump"-like deviation, Fig. ). The profile is anisotropic (SI: Fig. ) and implies the presence of oriented scattering features of ~10 nm in size, confirming the mesostructured nature of gypsum crystals . Noteworthy, these oriented nano-features partially disappear in developed bassanite crystals after 7200 s and in the re-precipitated gypsum after 15900 s (Fig. ).