title: Experimentally determined standard thermodynamic properties of synthetic MgSO(4)·4H(2)O (Starkeyite) and MgSO(4)·3H(2)O: a revised internally consistent thermodynamic data set for magnesium sulfate hydrates.
abstract: The enthalpies of formation of synthetic MgSO(4)·4H(2)O (starkeyite) and MgSO(4)·3H(2)O were obtained by solution calorimetry at T=298.15 K. The resulting enthalpies of formation from the elements are [Formula: see text] (starkeyite)=-2498.7±1.1 kJ·mol(-1) and [Formula: see text] (MgSO(4)·3H(2)O)=-2210.3±1.3 kJ·mol(-1). The standard entropy of starkeyite was derived from low-temperature heat capacity measurements acquired with a physical property measurement system (PPMS) in the temperature range 5 K<T<300 K: [Formula: see text] (starkeyite)=254.48±2.0 J·K(-1)·mol(-1). Additionally, differential scanning calorimetry (DSC) measurements with a Perkin Elmer Diamond DSC in the temperature range 270 K<T<300 K were performed to check the reproducibility of the PPMS measurements around ambient temperature. The experimental C(p) data of starkeyite between 229 and 303 K were fitted with a Maier-Kelley polynomial, yielding C(p)(T)=107.925+0.5532·T-1048894·T(-2). The hydration state of all Mg sulfate hydrates changes in response to local temperature and humidity conditions. Based on recently reported equilibrium relative humidities and the new standard properties described above, the internally consistent thermodynamic database for the MgSO(4)·nH(2)O system was refined by a mathematical programming (MAP) analysis. As can be seen from the resulting phase diagrams, starkeyite is metastable in the entire T-%RH range. Due to kinetic limitations of kieserite formation, metastable occurrence of starkeyite might be possible under martian conditions.
authors.affiliation: Institute of Geosciences, Mineralogy, Friedrich-Schiller University, D-07745 Jena, Germany. Klaus-Dieter.Grevel@rub.de
authors.forename: Klaus-Dieter
authors.initials: KD
authors.lastname: Grevel
authors.forename: Juraj
authors.initials: J
authors.lastname: Majzlan
authors.forename: Artur
authors.initials: A
authors.lastname: Benisek
authors.forename: Edgar
authors.initials: E
authors.lastname: Dachs
authors.forename: Michael
authors.initials: M
authors.lastname: Steiger
authors.forename: A Dominic
authors.initials: AD
authors.lastname: Fortes
authors.forename: Bernd
authors.initials: B
authors.lastname: Marler
mesh_terms: D002152:Calorimetry, Differential Scanning; D019277:Entropy; D007700:Kinetics; D008278:Magnesium Sulfate / Q000737:chemistry*; D015203:Reproducibility of Results; D013696:Temperature; D013816:Thermodynamics
chemical_list: D008278:Magnesium Sulfate