Temperature and humidity dependent formation of CaSO4·xH2O (x = 0...2) phases
Introduction
The Atacama Desert in Chile in its central hyperarid core is one of the driest areas on earth and, based on geological and mineralogical evidence it is likely, that hyperarid conditions have lasted for the past 3–4 million years (Hartley and Chong, 2002; Hartley et al., 2005; Dunai et al., 2005). In the hyperarid Atacama soils gypsum (CaSO4·2H2O) is the most dominant mineral (Ericksen, 1983; Chong Diaz, 1984; Ewing et al., 2006) and is believed to play, together with other compounds CaSO4 · xH2O (x = 0...2), an essential role concerning weathering of soil and rocks and soil dynamics (Ewing et al., 2006). In particular, a prominent feature in the Atacama Desert is the presence, formation (and disintegration) of so called ‘gypsum crusts’ (Ewing et al., 2006), which covers large areas with a firm surface. Constituents of ‘gypsum crust’ are, besides gypsum, bassanite (CaSO4 · ½H2O) and β-anhydrite (β-CaSO4) (Wierzchos et al., 2011; Flahaut et al., 2017), the two further calcium sulfate mineral phases. Our own X-ray diffraction phase analyses of surface crust samples corroborate the presence of the calcium sulfate minerals gypsum, bassanite and β-anhydrite, with metastable bassanite being a minor constituent, see Table 1.
Between these occurring mineral phases dehydration / rehydration processes can emerge depending on temperature, humidity (and pressure), which lead to phase transformations accompanied by substantial volume changes, but as well by solidification due to crystal growth processes. Here, the question arises concerning the requirements for these phase transformation processes to occur without the presence of liquid water (as it is given at hyperarid Atacama soil surfaces) but for typical temperatures and relative air humidity present in the Atacama Desert.
Because of the enormous technical importance of materials of the system CaSO4 - H2O this system has been investigated extensively. Disregarding high-pressure phases, at least eight duly structurally defined phases CaSO4 · xH2O, compiled in Table 2, are reported in literature:
Solely as technical products further CaSO4 · xH2O (x = 0…~0.5) phases are known (see, e.g., Gips, 2013). The subhydrates, i.e. phases with a water content per formula unit differing from 0, 0.5 or 2, were exclusively observed under laboratory conditions and are depicted as interim products that can occur during CaSO4 or CaSO4 · ½ H2O rehydration to gypsum (Bezou et al., 1995; Schmidt et al., 2011). The variety of subhydrate phases is regarded to result from the ability of the bassanite-type structure to accommodate various amounts of H2O molecules, dependent on the relative humidity of formation conditions (Schmidt et al., 2011).
Due to their technical and scientific significance processes of transformations (hydration and dehydration) between the above phases have been studied within a broad range of conditions, such as temperature (e.g., McAdie, 1964; Ball and Norwood, 1969; Christensen et al., 1985; Putnis et al., 1990; Badens et al., 1998; Chang et al., 1999; Prasad et al., 2005; Jordan and Astilleros, 2006; Lebron et al., 2009; Ballirano and Melis, 2009), atmospheric pressure (e.g., Molony and Ridge, 1968; McConnell et al., 1987; Carbone et al., 2008; Mirwald, 2008), further added chemical components (e.g., Hardie, 1967; Cody and Hull, 1980; Charola et al., 2007; Singh and Middendorf, 2007; Wilson and Bish, 2011; Ossorio et al., 2014) or partial pressure of H2O (e.g., McAdie, 1964; Ball and Norwood, 1969; Badens et al., 1998; Oetzel et al., 2000; Jordan and Astilleros, 2006; Lebron et al., 2009; Wilson and Bish, 2011; Robertson and Bish, 2013), applying a large number different experimental techniques (for a review see, e.g., Freyer and Voigt, 2003). However, despite of the large number of different studies, the results vary to a large extend, even for studies carried out under approximately similar experimental conditions, and in fact, there is still uncertainty concerning the mechanism of dehydration and hydration, the kinetics and number of steps as well as concerning the type of phases produced during dehydration and hydration. Consequently, emphasis needs to be placed on the parameters controlling hydration and dehydration processes of CaSO4 · xH2O phases, in particular, a simultaneous control of temperature, humidity and sample constitution (particularly grain size, sample thickness, porosity) by in situ measurements. The aim of this work is to investigate dehydration (and hydration) processes of CaSO4·xH2O (x = 0...2) phases without the presence of liquid H2O in the system, using in situ humidity- and temperature-controlled powder X-ray diffraction (powder-XRD). Applied temperature and humidity parameters are inspired by the extremes of climatic conditions present in the Atacama Desert, in order to constrain the stability of CaSO4 · xH2O phases under these conditions. In this study the number of variable parameters is limited to time, temperature, relative air humidity and sample particle size, keeping chemical composition fixed, in order to set a basis of data for the solid state phases of the pure system CaSO4 - H2O, before in the future extending the system towards natural sample compositions. Working with samples of chemical compositions like those found in the Atacama Desert will be subject to upcoming studies. The experimental data derived from the present study can give a contribution to answer the question, whether dehydration and hydration transformations of CaSO4·xH2O phases and their kinetics can occur over diurnal or seasonal cycles under hyperarid conditions of the Atacama Desert and therefore are either negligible or abundant phenomena, which can lead to formation or disintegration of ‘gypsum crusts’.
Section snippets
Materials
Calcium sulfate dihydrate from Emsure Merck (CAS 10101–41–4, ≤0.01% fractions insoluble in HCl, precipitated for analysis), was treated with demineralized water at 45 °C for one day to rehydrate any residual traces of other calcium sulfates to gypsum. The recrystallized material was ground in an agate mortar and sieved into fractions of <50 μm for particle size independent experiments and into fractions of <20 μm, 20–25 μm, 25–32 μm and 32–50 μm for particle size dependent experiments. As
Results and discussion
In powder X-ray diffraction the differentiation between the three mineral phases gypsum, bassanite and β-anhydrite is straightforward, as Fig. 1 demonstrates, due to the rather different crystal structures and powder patterns. While gypsum crystallizes with a monoclinic (space group I2/a) layer structure of alternating Ca + [SO4]2− and H2O layers (e.g., Atoji and Rundle, 1958) the monoclinic (space group I2), pseudo-trigonal structure of bassanite consists of six-membered interlinked distorted
Conclusion
The results of the present study of temperature and humidity dependent formation of CaSO4·xH2O (x = 0...2) phases by dehydration and rehydration processes show, that gypsum dehydration to bassanite (and further on to β-anhydrite during long time range) can proceed already at rather low temperatures of 80 °C and even lower, however, the conversion rate decreases rapidly with lower temperature, leading to very long time for gypsum dehydration to take place. Rehydration of bassanite to gypsum
Declaration of Competing Interest
None.
Acknowledgment
The authors thank Dr. D. Hoffmeister, University of Cologne, SFB 1211, project Z02, for the supply of data concerning soil surface temperatures at the meteorological stations in the Atacama Desert. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 268236062 – SFB 1211 (project C04).
References (73)
- et al.
Study of gypsum dehydration by controlled transformation rate thermal analysis (CRTA)
J. Solid State Chem.
(1998) - et al.
Investigation of the crystal structure of γ-CaSO4, CaSO4 · 0.5 H2O, and CaSO4 · 0.6 H2O by powder diffraction methods
J. Solid State Chem.
(1995) - et al.
Application of thermo-Raman spectroscopy to study dehydration of CaSO4·2H2O and CaSO4·0.5H2O
Mater. Chem. Phys.
(1999) - et al.
Rain infiltration and crust formation in the extreme arid zone of the Atacama desert, Chile
Planet. Space Sci.
(2010) - et al.
A threshold in soil formation at earth's arid-hyperarid transition
Geochim. Cosmochim. Acta
(2006) - et al.
Remote sensing and in situ mineralogic survey of the Chilean salars: An analog to mars evaporate deposits?
Icarus
(2017) - et al.
The gypsum-anhydrite paradox revisited
Chem. Geol.
(2014) - et al.
Constraints on the distribution of CaSO4·nH2O phases on mars and implications for their contribution to the hydrological cycle
Icarus
(2013) - et al.
Calcium sulphate hemihydrate hydration leading to gypsum crystallization
Prog. Cryst. Growth Charact. Mater.
(2007) Bundesverband der Gipsindustrie e.V
(2013)
Diffrac Suite 5.0 (containing Diffrac Eva 3.0 and Diffrac Topas 5.0)
Powder Diffraction FileTM
Calcium Sulfat Subhydrat, CaSO4.0,8H2O
Acta Cryst. C
Bestimmung der Kristallstruktur von CaSO4(H2O)0,5 mit Röntgenbeugungsmethoden und mit Potentialprofil-Rechnungen
Z. Kristallogr.
Crystal-structure analysis of four mineral samples of anhydrite, CaSO4, using synchrotron high-resolution powder X-ray diffraction data
Powder Diffract.
Neutron diffraction study of gypsum, CaSO42H2O
J. Chem. Phys.
Kinetics of phase change. II Transformation-time relations for random distribution of nuclei
J. Chem. Phys.
Studies of the system calcium sulphate-water. part i kinetics of dehydration of calcium sulphate dihydrate
J. Chem. Soc. A
Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction
Phys. Chem. Miner.
The monoclinic I2 structure of bassanite, calcium sulphate hemihydrate (CaSO4 · 0.5H2O)
Eur. J. Mineral.
Compt. Rend. Acad. Sci. Paris, Ser. II
Redetermination of the crystal structure of calcium sulphate dihydrate, CaSO4·2(H2O)
Z. Kristallogr. NCS
Water of crystallization in the CaSO4.0.67H2O and CaSO4.0.5H2O structures
Russ. J. Inorg. Chem.
X-Ray diffraction investigation of CaSO4.0.67H2O
Russ. J. Inorg. Chem.
Phase transformations in the dehydration of CaSO4.2H2O
Russ. J. Inorg. Chem.
Kinetics of gypsum dehydration at reduced pressure: an energy dispersive X-ray diffration study
Eur. J. Mineral.
Gypsum: a review of its role in the deterioration of building materials
Environ. Geol.
The crystal structure of anhydrite (CaSO4)
Acta Cryst
Die Salare in Nordchile
Geotecton. Res.
A time-resolved neutron powder diffraction investigation of the hydration of CaSO4.½D2O and of the dehydration of CaSO4.2D2O
J. Appl. Crystallogr.
Formation and transformation of five different phases in the CaSO4-H2O system: Crystal structure of the subhydrate β-CaSO4·0.5H2O and soluble anhydrite CaSO4
Chem. Mater.
A new calcium sulfate hemi-hydrate
Dalton Trans.
Experimental growth of primary anhydrite at low temperatures and water salinities
Geology
A refinement of the crystal structure of gypsum CaSO4.2H2O
Acta Cryst. B
Structure and microstructure of gypsum and its relevance to Rietveld quantitative phase analyses
Powder Diffract.
Oligocene-miocene age of aridity in the Atacama desert revealed by exposure dating of erosion-sensitive landforms
Geology
Cited by (13)
Long-term environmental monitoring for preventive conservation of external historical plasterworks
2022, Journal of Building EngineeringCitation Excerpt :In this regard, Winkler & Wilhelm (1970) indicated the relationship that must exist between ambient temperature and relative humidity to transform gypsum (CaSO4·2H2O) into bassanite (CaSO4·0.5H2O) [31], In this case, the material is likely to lose stability if exposure to these unfavorable environmental conditions is prolonged over time [32] (Fig. 1-A, 1-B). However, this transformation is part of a reversible process in plasterwork, which means that a rehydration would allow the recovery of the initial properties as long as the dehydration had not been prolonged in time until it remained constant, due to a change in climatic conditions, causing the mechanical weakening of the plasterwork [32]. Additionally, gypsum has low hygroscopicity [33,34] and medium-low solubility [35] that do not represent a direct risk in the preservation of plasterwork [36].
Established soil science methods can benefit the construction industry when determining gypsum content
2021, Cleaner Engineering and TechnologyCitation Excerpt :The transformation of bassanite in to gypsum has been studied at nanoscale by Saha et al. (2012). Ritterbach and Becker (2020) stress that the kinetics of gypsum dehydration and bassanite rehydration are dependent of particle size, and have shown that bassanite can re-hydrate to gypsum with high relative air humidity of 95% without liquid water. False records of anhydrite and/or bassanite can be obtained when observed under a microscope.
Whitepaper: Earth – Evolution at the dry limit
2020, Global and Planetary ChangeCitation Excerpt :To quantitatively interpret topographic and hydrologic patterns concerning long-term and transient regional differences in runoff and erosion (Bartz et al., in 2020; Binnie et al., 2020, May et al., 2020; Medialdea et al., 2020; Mohren et al., 2020; Ritter et al., 2018b; Walk et al., 2020); To investigate the formation of calcium sulphates, chlorides and nitrates to better understand their potential for the reconstruction of humidity changes and role for landscape modulation (Ritterbach and Becker, 2020; Voigt et al., 2020). Some methodological challenges and tasks have to be met by the CRC 1211.