Temperature Fluctuation‐Driven Salt Crystallization in Salt Lakes and Nearby Sands—Lab Emulations

Salt crystallization is common in nature. While evaporation‐driven salt crystallization is well studied, crystallization driven by temperature fluctuation has received less attention. This study utilizes in situ micro‐CT to track Na 2 SO 4 crystallization in bulk solution and porous media under temperature fluctuations, emulating scenarios in salt lakes and nearby sandy soils. The 3D high‐resolution microscopic results show that, in bulk solution, crystallization begins with random nucleation, followed by crystal settling, which forms crystal accumulations at the bottom. As temperature increases, the accumulated Na 2 SO 4 ·7H 2 O transforms into anhydrous Na 2 SO 4 and its saturated solution, according to the local salt‐water ratio and the phase diagram. Upon re‐cooling, reprecipitation intensifies crystal accumulation at the bottom. In porous media, particles affect not only nucleation, with rough sand promoting more widespread nucleation than smooth glass beads, but also nuclei settling, both of which produce a more uniform crystal distribution. Nevertheless, gravity still facilitates crystal accumulation at the bottom. The temperature increase and re‐cooling cycle also produce anhydrous Na 2 SO 4 and intensified crystal accumulation at the bottom. In all cases, the widely neglected metastable Na 2 SO 4 ·7H 2 O persistently precipitates under temperature fluctuations and can coexist with stable anhydrous Na 2 SO 4 . Both systems reveal a new mechanism for salt accumulation.