title: 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.
authors.affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.|German Research Centre for Geosciences, GFZ, 14473 Potsdam, Germany.
authors.forename: Tomasz M
authors.initials: TM
authors.lastname: Stawski
authors.affiliation: LEC, IACT, CSIC-UGR, E-18100 Armilla, Spain.|Structural Biology Brussels, VUB, 1050 Brussels, Belgium.|CNRS, ISTerre, F-38041 Grenoble, France.
authors.forename: Alexander E S
authors.initials: AE
authors.lastname: van Driessche
authors.affiliation: LEC, IACT, CSIC-UGR, E-18100 Armilla, Spain.
authors.forename: Mercedes
authors.initials: M
authors.lastname: Ossorio
authors.affiliation: Nano-Science Center; Department of Chemistry; University of Copenhagen, DK 2100 Copenhagen, Denmark.
authors.forename: Juan
authors.initials: J
authors.lastname: Diego Rodriguez-Blanco
authors.affiliation: German Research Centre for Geosciences, GFZ, 14473 Potsdam, Germany.
authors.forename: Rogier
authors.initials: R
authors.lastname: Besselink
authors.affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.|German Research Centre for Geosciences, GFZ, 14473 Potsdam, Germany.
authors.forename: Liane G
authors.initials: LG
authors.lastname: Benning