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Nucleation pathway of calcium sulfate hemihydrate (bassanite) from solution: Implications for calcium sulfates on Mars

DOI: 10.1021/acs.jpcc.0c01041 DOI Help

Authors: Tomasz M. Stawski (Federal Institute for Materials Research and Testing (BAM)) , Rogier Besselink (German Research Centre for Geosciences, GFZ; Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS) , Konstantinos Chatzipanagis (German Research Centre for Geosciences, GFZ; National Hellenic Research Foundation) , Jorn Hovelmann (German Research Centre for Geosciences, GFZ; Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS) , Liane G. Benning (German Research Centre for Geosciences, GFZ; Freie Universität Berlin) , Alexander E. S. Van Driessche (Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: The Journal Of Physical Chemistry C

State: Published (Approved)
Published: March 2020
Diamond Proposal Number(s): 16256

Abstract: CaSO4 minerals (i.e. gypsum, anhydrite and bassanite) are widespread in natural and industrial environments. During the last several years, a number of studies have revealed that nucleation in the CaSO4-H2O system is non-classical, where the formation of crystalline phases involves several steps. Based on these recent insights we have formulated a tentative general model for calcium sulfate precipitation from solution. This model involves primary species that are formed through the assembly of multiple Ca2+ and SO42- ions into nanoclusters. These nanoclusters assemble into poorly ordered (i.e. amorphous) hydrated aggregates, which in turn undergo ordering into coherent crystalline units. The thermodynamic (meta)stability of any of the three CaSO4 phases is regulated by temperature, pressure and ionic strength with gypsum being the stable form at low temperatures and low to medium ionic strengths, and anhydrite the stable phase at high temperatures and lower temperature at high salinities. Bassanite is metastable across the entire phase diagram but readily forms as the primary phase at high ionic strengths across a wide range of temperatures, and can persist up to several months. Although the physicochemical conditions leading to bassanite formation in aqueous systems are relatively well established, nanoscale insights into the nucleation mechanisms and pathways are still lacking. To fill this gap, and to further improve our general model for calcium sulfate precipitation, we conducted in situ scattering measurements at small- and wide-angles (SAXS/WAXS) and complemented these with in situ Raman spectroscopic characterization. Based on these experiments we show that the process of formation of bassanite from aqueous solutions is very similar to the formation of gypsum: it involves the aggregation of small primary species into larger disordered aggregates, only from which the crystalline phase develops. These data thus confirm our general model of CaSO4 nucleation and provide clues to explain the abundant occurrence of bassanite on the surface of Mars (and not on the surface of Earth).

Subject Areas: Chemistry, Earth Science, Physics

Instruments: I22-Small angle scattering & Diffraction

Other Facilities: ALBA; ESRF

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