Abstract
The formation pathways of gypsum remain uncertain. Here, using truly in situ and fast time-resolved small-angle X-ray scattering, we quantify the four-stage solution-based nucleation and growth of gypsum (CaSO4·2H2O), an important mineral phase on Earth and Mars. The reaction starts through the fast formation of well-defined, primary species of <3 nm in length (stage I), followed in stage II by their arrangement into domains. The variations in volume fractions and electron densities suggest that these fast forming primary species contain Ca–SO4-cores that self-assemble in stage III into large aggregates. Within the aggregates these well-defined primary species start to grow (stage IV), and fully crystalize into gypsum through a structural rearrangement. Our results allow for a quantitative understanding of how natural calcium sulfate deposits may form on Earth and how a terrestrially unstable phase-like bassanite can persist at low-water activities currently dominating the surface of Mars.
Highlights
The formation pathways of gypsum remain uncertain
Wang et al.[12] showed by transmission electron microscopy (TEM) imaging of time-resolved quenched samples that the formation of gypsum was preceded by bassanite, an amorphous calcium sulfate phase was the first phase to nucleate from solution before bassanite
We investigated the formation of solid CaSO4 phases from various supersaturated solutions with initial concentrations between 50 and 150 mmol l À 1 and at temperatures between 12 and 40 °C
Summary
The formation pathways of gypsum remain uncertain. Here, using truly in situ and fast timeresolved small-angle X-ray scattering, we quantify the four-stage solution-based nucleation and growth of gypsum (CaSO4 Á 2H2O), an important mineral phase on Earth and Mars. True in situ and time-resolved characterization of the formation reactions in solution could not be achieved in any of the above studies and this lead to the current uncertainty about the formation pathway(s) of gypsum To overcome this impasse, we performed synchrotron-based in situ and highly temporally resolved X-ray small- and wideangle scattering experiments (SAXS/WAXS) in a way that we could follow all nucleation, growth and transformation reactions in solutions. These entities constitute the primary building blocks, which through aggregation, self-assembly and a structural rearrangement transform to gypsum Based on these new insights we propose a non-classical four-stage pathway for calcium sulfate formation from homogenous solution
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