Nucleation Pathway of Calcium Sulfate Hemihydrate (Bassanite) from Solution: Implications for Calcium Sulfates on Mars

  • 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 nonclassical, 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 being the stable phase at high temperatures and at lower temperature forCaSO4 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 nonclassical, 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 being the stable phase at high temperatures and at lower temperature for 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-angle X-ray scattering and wide-angle X-ray scattering 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).zeige mehrzeige weniger

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Autor*innen:Tomasz StawskiORCiD, R. Besselink, K. Chatzipanagis, J. Hövelmann, L. G. Benning, E. S. Van Driessche
Dokumenttyp:Zeitschriftenartikel
Veröffentlichungsform:Verlagsliteratur
Sprache:Englisch
Titel des übergeordneten Werkes (Englisch):The Journal of Physical Chemistry C
Jahr der Erstveröffentlichung:2020
Organisationseinheit der BAM:6 Materialchemie
6 Materialchemie / 6.3 Strukturanalytik
Verlag:American Chemical Society
Jahrgang/Band:124
Ausgabe/Heft:15
Erste Seite:8411
Letzte Seite:8422
DDC-Klassifikation:Technik, Medizin, angewandte Wissenschaften / Ingenieurwissenschaften / Ingenieurwissenschaften und zugeordnete Tätigkeiten
Freie Schlagwörter:Bassanite; Calcium sulfate; Gypsum' SAXS; Nucleation
Themenfelder/Aktivitätsfelder der BAM:Material
Material / Nano
DOI:10.1021/acs.jpcc.0c01041
Verfügbarkeit des Dokuments:Datei im Netzwerk der BAM verfügbar ("Closed Access")
Datum der Freischaltung:10.06.2020
Referierte Publikation:Ja
Datum der Eintragung als referierte Publikation:10.06.2020
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