Calcium selenate
| Identifiers | |
|---|---|
| |
| Properties | |
| CaSeO4 | |
| Molar mass | 183.04 g/mol |
| Appearance | Colorless crystalline solid |
| Soluble in water | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
Calcium selenate is an inorganic compound of calcium and the selenate ion, with the chemical formula CaSeO4. It forms several crystalline polymorphs and hydrated phases. The dihydrate, CaSeO4·2H2O, is isomorphous with gypsum, CaSO4·2H2O.[1]
Preparation
[edit]Calcium selenate dihydrate can be precipitated from aqueous solutions of a soluble calcium salt and a soluble selenate, for example from calcium chloride and sodium selenate:[1]
- CaCl2 + Na2SeO4 + 2 H2O → CaSeO4·2H2O + 2 NaCl
Anhydrous calcium selenate polymorphs can be prepared by dehydration or by hydrothermal crystallization under controlled conditions. Orthorhombic and monoclinic CaSeO4 have been crystallized at about 210 °C, whereas the tetragonal modification has been obtained at approximately 70 °C.[2]
Properties
[edit]Structure
[edit]Three polymorphs of anhydrous CaSeO4 have been structurally characterized.[2]
The orthorhombic modification crystallizes in space group Cmca (No. 64), with lattice parameters a = 7.192 Å, b = 14.404 Å and c = 6.398 Å, and eight formula units per unit cell. It represents a distinct structure type with structural similarities to both anhydrite and zircon.[2]
The monoclinic modification crystallizes in space group P21/n (No. 14), with a = 6.860 Å, b = 7.086 Å, c = 6.692 Å and β = 104.21°, with Z = 4. It is isotypic with the monazite structure.[2]
The tetragonal modification crystallizes in space group I41/a (No. 88), with a = 5.054 Å and c = 11.678 Å, and has the scheelite structure type.[2]
In all three polymorphs, selenium occurs in SeO42− tetrahedra, while calcium is eight-coordinate.[2]
Hydrates
[edit]Calcium selenate forms several hydrates, of which the dihydrate is the best characterized.
CaSeO4·2H2O is monoclinic and isomorphous with gypsum. Because sulfate and selenate ions have similar size and geometry, CaSO4·2H2O and CaSeO4·2H2O can form solid solutions, although a substantial miscibility gap exists at room temperature.[1]
The solubility of the dihydrate decreases with increasing temperature. A critical review of thermodynamic data selected a solubility of
- 0.380 ± 0.025 mol CaSeO4 kg−1 H2O
at 298.15 K.[3]
Partial dehydration produces calcium selenate subhydrates. A phase nominally described in older literature as CaSeO4·0.5H2O was subsequently reinvestigated by single-crystal and synchrotron X-ray diffraction. Material produced hydrothermally from the dihydrate at 463 K was found to have the composition CaSeO4·0.625H2O.[4]
This subhydrate crystallizes in the trigonal space group P3221. Its structure is related to that of bassanite, with water molecules occupying channels in the calcium–selenate framework.[4]
Thermal behaviour
[edit]On heating, calcium selenate dihydrate loses water and forms lower hydrates or anhydrous CaSeO4. The dehydration behaviour is closely analogous to that of calcium sulfate, although differences in phase stability arise from the larger SeO42− ion.[1][4]
The thermal decomposition of calcium selenate at higher temperatures has been studied by thermogravimetric and differential thermal analysis.[5]
References
[edit]- 1 2 3 4 Fernández-González, Angeles; Andara, Angel; Alía, José María; Prieto, Manuel (2006). "Miscibility in the CaSO4·2H2O–CaSeO4·2H2O system: Implications for the crystallisation and dehydration behaviour". Chemical Geology. 225 (3–4): 256–265. doi:10.1016/j.chemgeo.2005.08.019.
- 1 2 3 4 5 6 Pristacz, Helmut; Talla, Dominik; Preuschl, F.; Giester, Gerald; Wildner, Manfred (2014). "Crystal structures of three CaSeO4 polymorphs and their mineralogical relationships". Neues Jahrbuch für Mineralogie - Abhandlungen. 191 (2): 215–223. doi:10.1127/0077-7757/2014/0257.
- ↑ Olin, Åke; Noläng, Bengt; Öhman, Lars-Olof; Osadchii, Evgeniy; Rosén, Erik (2005). Chemical Thermodynamics of Selenium. Chemical Thermodynamics. Vol. 7. OECD Nuclear Energy Agency.
- 1 2 3 Fritz, Susanne; Schmidt, Horst; Paschke, Iris; Magdysyuk, Oxana V.; Dinnebier, Robert E.; Freyer, Daniela; Voigt, Wolfgang (2011). "CaSeO4·0.625H2O—water channel occupation in a bassanite related structure". Acta Crystallographica Section B. 67 (4): 293–301. doi:10.1107/S0108768111018544.
- ↑ de Ávila Agostini, P. R.; de Castilho Agostini, E.; Giolito, I.; Ionashiro, M. (1989). "Thermal decomposition of magnesium and calcium selenates". Thermochimica Acta. 145: 367–371.