Formation of calcium sulfate through the aggregation of sub-3 nanometre primary species

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.

Not just fractal surfaces, but surface fractal aggregates: Derivation of the expression for the structure factor and its applications

“…28 In our in situ and realtime scattering data on the nucleation and growth of CaSO 4 solids from supersaturated aqueous solution, we observed very characteristic surface fractal morphologies. These data sets were the initial inspiration to draft the surface fractal aggregate concept, which we now expanded and formalized in this work.…”

“…6, we show a typical in situ small-angle X-ray scattering curve obtained from a 75 mmol/l CaSO 4 supersaturated solution after 150 s at 21 • C. In contrast to the assumptions made in our derivation, for the case of the experimental CaSO 4 data, the primary particles were found not to be spherical, but rather cylindrical in shape. 28 Thus, to fit the (5) and (15), but by using a cylindrical form factor (variables R and L) 42 instead of the Guinier approximation from Eq. (20).…”

“…25 This is particularly relevant if we consider that recent studies have pointed out that, for example, nucleation is driven by local density fluctuations, 26,27 and that due to these fluctuations the local particle concentration can significantly increase resulting in much decreased diffusion lengths. In a recent work 28 we showed that, during the formation of gypsum (CaSO 4 ·2H 2 O), surface fractal aggregates made of sub-3 nm primary species constituted a crucial step in the process, without which nucleation nor further growth could proceed. This insight was gained from experimental in situ and time-resolved scattering evidence and showed that the appearance of surface fractal aggregates was indeed preceded by the formation of domains of increased local number density of the primary species.…”

Physicochemical and Additive Controls on the Multistep Precipitation Pathway of Gypsum

“…In a previous study, using in situ SAXS/WAXS, we observed the initial stages of calcium sulphate nucleation and found that nanosized primary particles are formed immediately after a supersaturated solution is created. These particles aggregate and transform/re-organize into gypsum [12]. Building upon this initial study, we again used in situ SAXS/WAXS to follow the precipitation behavior of calcium sulphate phase(s) over a broad range of temperatures (4-40 • C) and initial calcium sulphate concentrations (50-150 mM).…”

Using Combined TEM, Raman, XRD, and VNIR techniques to Investigate Secondary Phase Formation and Textural Relationships in Brine + Jarosite Experiments

“…Other workers have suggested that Ca-sulfate minerals form via complex nucleation, aggregation, and growth processes [4] that may also be affected by hydrologic flow rates, as well as different mineral substrates present in the system. We conducted short-term (22 day), low temperature (20℃) flow-through and batch reactor experiments with different brine concentrations and mineral substrates -CaCl2 brines + jarosite and MgSO4/MgCl2 brines + calcite (CaCO3) -to test Dixon et al's hypothesis in addition to the hypothesis that varying the mineral substrate would affect reaction products.…”