Water-rich glassy states from mixed sulfate salts

Abstract Glasses exhibit unique mechanical and physicochemical properties distinct from crystalline solids, with relevance to biomineralization, solid-state physics, pharmaceutical chemistry, and planetary science. However, despite their widespread occurrence and fundamental importance, simple chemical rules that predict when an amorphous rather than a crystalline phase forms from solution remain scarce. Here, we exploit the complexity of mixed-salt crystallization to show the spontaneous formation of amorphous glassy states from a wide variety of mixed sulfate solutions. Isothermal evaporation of sulfate solutions containing specific ratios of mono- and multi-valent cations yields an amorphous glassy state. We find that this state is soft, viscoelastic, X-ray amorphous, and retains up to 40 wt% water. Spectroscopic analysis further reveals that disordered and immobilized ionic species trap bulk-like water. By modulating composition and ambient relative humidity, we show that the temporal stability of the glassy state can be tuned from seconds to months. Our findings help uncover fundamental mechanisms governing the formation and stability of amorphous phases, with direct relevance to complex biomineralization processes on Earth, and offer a compelling explanation for the widespread occurrence of amorphous sulfate materials in Martian mineralogy. Moreover, the ability to precisely control the synthesis of these metastable amorphous states and tune their lifetime opens opportunities for the design of next-generation materials, particularly in advanced drug delivery applications.