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

Stawski TM, van Driessche AE, Ossorio M, Diego Rodriguez-Blanco J, Besselink R, Benning LG - Nat Commun (2016)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.

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.

No MeSH data available.


Related in: MedlinePlus

Electron densities for CaSO4pseudo-phases based on SAXS.The values as a function of time were calculated based on the corresponding volume fractions of the known crystalline phases: (a) not taking into account the bulk solubility; (b) taking into account bulk solubility in pure water; (c) taking into account bulk solubility in a 100-mmol l−1 NaCl solution. The horizontal bands indicate the range of electron densities expected for each given CaSO4 polymorph. Details of how the electron density values were calculated can be found in Supplementary Note 9 and Supplementary Table 2.
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f4: Electron densities for CaSO4pseudo-phases based on SAXS.The values as a function of time were calculated based on the corresponding volume fractions of the known crystalline phases: (a) not taking into account the bulk solubility; (b) taking into account bulk solubility in pure water; (c) taking into account bulk solubility in a 100-mmol l−1 NaCl solution. The horizontal bands indicate the range of electron densities expected for each given CaSO4 polymorph. Details of how the electron density values were calculated can be found in Supplementary Note 9 and Supplementary Table 2.

Mentions: Inexorably the question arises about the nature of these primary species and their evolution through time. Monitoring the φVpart(Δρ)2, and the normalized φ(Δρ)2 pre-factors (Fig. 3b,f), allowed us to analyse the reaction progress as a function of time. However, specific information concerning the electron density, Δρ, and volume fraction, φ, of the forming phase is not independently accessible because the pre-factors are the product of these two components. Nevertheless, the scattered intensity is expressed in absolute units, and thus the changes in the φ(Δρ)2 pre-factor in relation to known CaSO4 polymorphs and their expected volume fractions can be estimated. This way we can correlate our scattering data with thermodynamic solubility data, and identify the formed phase(s) based on their electron densities. We evaluated the predicted volume fractions, φ, for each possible CaSO4 phase by considering the original concentration of Ca2+ and SO42− ions in solution and using the various bulk solubility of each phase calculated with PHREEQC (ref. 32). Using the values of φ , we calculated the corresponding electron densities from our SAXS data (pseudo-phases, Supplementary Note 9 and Supplementary Table 2), and compared them to the actual electron densities of each of the three CaSO4 phases (Fig. 4).


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

Stawski TM, van Driessche AE, Ossorio M, Diego Rodriguez-Blanco J, Besselink R, Benning LG - Nat Commun (2016)

Electron densities for CaSO4pseudo-phases based on SAXS.The values as a function of time were calculated based on the corresponding volume fractions of the known crystalline phases: (a) not taking into account the bulk solubility; (b) taking into account bulk solubility in pure water; (c) taking into account bulk solubility in a 100-mmol l−1 NaCl solution. The horizontal bands indicate the range of electron densities expected for each given CaSO4 polymorph. Details of how the electron density values were calculated can be found in Supplementary Note 9 and Supplementary Table 2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4821993&req=5

f4: Electron densities for CaSO4pseudo-phases based on SAXS.The values as a function of time were calculated based on the corresponding volume fractions of the known crystalline phases: (a) not taking into account the bulk solubility; (b) taking into account bulk solubility in pure water; (c) taking into account bulk solubility in a 100-mmol l−1 NaCl solution. The horizontal bands indicate the range of electron densities expected for each given CaSO4 polymorph. Details of how the electron density values were calculated can be found in Supplementary Note 9 and Supplementary Table 2.
Mentions: Inexorably the question arises about the nature of these primary species and their evolution through time. Monitoring the φVpart(Δρ)2, and the normalized φ(Δρ)2 pre-factors (Fig. 3b,f), allowed us to analyse the reaction progress as a function of time. However, specific information concerning the electron density, Δρ, and volume fraction, φ, of the forming phase is not independently accessible because the pre-factors are the product of these two components. Nevertheless, the scattered intensity is expressed in absolute units, and thus the changes in the φ(Δρ)2 pre-factor in relation to known CaSO4 polymorphs and their expected volume fractions can be estimated. This way we can correlate our scattering data with thermodynamic solubility data, and identify the formed phase(s) based on their electron densities. We evaluated the predicted volume fractions, φ, for each possible CaSO4 phase by considering the original concentration of Ca2+ and SO42− ions in solution and using the various bulk solubility of each phase calculated with PHREEQC (ref. 32). Using the values of φ , we calculated the corresponding electron densities from our SAXS data (pseudo-phases, Supplementary Note 9 and Supplementary Table 2), and compared them to the actual electron densities of each of the three CaSO4 phases (Fig. 4).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.

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.

No MeSH data available.


Related in: MedlinePlus