THERMODYNAMICS AND CRYSTALLOGRAPHIC COMPLEXITY OF SODIUM URANYL SULFATE MINERALS WITH UNIQUE TOPOLOGIES
This project has undertaken the synthesis, characterization and calorimetric analysis of the White Canyon suite of sodium uranyl sulfates and their closely-related synthetic phases. Pure synthetic analogs were crystallized through evaporation in isotherm ovens from solutions of U, Na and S. The resulting phases were fully characterized with single-crystal X-ray diffraction (SC-XRD), powder X-ray diffraction (PXRD), scanning electron microscope (SEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), and thermogravimetric analysis (TGA). The ground crystals were pressed into ~5 mg pellets and dropped into an AlexSys high-temperature 3D Calvet-type calorimeter to measure the enthalpy of solution, from which the enthalpy of formation (ΔH°f ) was determined4. Finally, the structural complexity of each phase was calculated using the ToposPro package5,6.
Our results have found that structural complexity is strongly correlated with the ΔH°f of the phases, implying the formation mechanism is energetic rather than entropic in nature. Continuing work will focus on the synthesis of additional sodium uranyl sulfates and determining whether the correlation between complexity and stability extends to other uranium groups.
References:
1IMA Mineral List Database. Available online: http://rruff.info/ima/ (accessed 12 August 2018).
2Olds, et al. Mineralogical Magazine. (2018) 1-30.
3J. Plášil. (2014) J. Geosci. 59: 99-114.
4A. Navrotsky. (2014) J. Am. Ceram. Soc. 97: 3349-3359.
5V.A. Blatov et al. (2014) Cryst. Growth Des. 14: 3576–3586.
6S.V. Krivovichev. (2016) Acta Cryst. B72: 274-276.