To specifically measure the solubility changes of calcium sulfate phases during temperature variations, we employed Raman spectroscopy to monitor the cyclic formation of gypsum and its subsequent transformation to bassanite (Fig. , SI: Materials and Methods). Through the analysis of time-resolved Raman spectra, we were able to determine the variations in the concentration of dissolved "free" sulfate in the solution. This was achieved by tracking the peak of the ν1 mode of aqueous sulfate at ~980 cm -1 (Fig. , Fig. ). Using our Raman setup we performed an experiment analogous to the one with scattering in Fig. , in which after mixing the stock solutions of CaCl 2 and Na 2 SO 4 (0.2 M CaSO 4 in 4.8 M NaCl), gypsum precipitated (Fig. , Fig. ). The time scales of the spectroscopic experiment differ from the scattering experiment due to specific constraints of each experimental setup (SI: Materials and Methods, Table ). However, the essential aspects of the gypsum-bassanite transformations are analogous and directly comparable between the two approaches (Fig. ). Upon the precipitation of gypsum, the sulfate concentration gradually decreased during the initial 1800 s to ~39 mM, after which it remained stable until the temperature was raised to T = 90 °C (Fig. ). The plateau in sulfate concentration at T = 35 °C indicated that the system had approached a state of thermodynamic equilibrium. However, from the scattering trends observed in Fig. , it is evident that gypsum crystals continued to evolve even before the temperature increase (Fig. , stage I). As the solution started to cool down to T = 35 °C, bassanite began to dissolve, increasing the sulfate concentration to ~41 mM. After a prolonged induction time (relative ~48000 s), gypsum started to crystallize, causing the sulfate concentration to decrease to ~37 mM. These two reduction steps in sulfate concentration are consistent with our scattering data (Fig. , Fig. and Fig. ), where we observed the morphological evolution of bassanite crystals induced by cooling before any gypsum crystals appeared. In fact, this dissolution of bassanite accompanied by an increase in sulfate concentration, is analogous to the reverse gypsum-bassanite transformation. The key difference lies in the kinetic rates: gypsum dissolves rapidly while bassanite dissolves slowly. This difference in dissolution rates may explain why secondary gypsum crystals exhibit a higher degree of mesoscale ordering compared to the initial crystals (Fig. ). Namely, the gradual increase in saturation in the presence of an interface (i.e. dissolving bassanite crystals) would promote the formation of fewer nuclei, slower growth, and consequently, larger and better-ordered gypsum crystals, which aligns with our observations. When gypsum became the dominant phase, the sulfate concentration increased slightly, and with only gypsum remaining, the concentration of sulfate equilibrated at ~41 mM. When the same solution was heated again to T = 90 °C, a very similar trend in the sulfate concentration profile as the one observed during the first heating stage was observed (Fig. ).