THERMODYNAMIC AND CRYSTALLOGRAPHIC INVESTIGATION OF SODIUM URANYL SULFATE MINERALS WITH UNIQUE TOPOLOGIES

Uranyl sulfate minerals are a structurally diverse group commonly found in localities under acid mine conditions¹. The number of known species in this group has more than doubled in the past ten years², with 23 of these new species discovered within the White Canyon mines of southeastern Utah^2,3. These uranyl sulfates vary in molar ratio of M:U:S (M = Na, NH₄, Mg, Al, Fe, Y or no cation), in water and hydroxide content, and in structural complexity⁴. The thermodynamic properties of these new minerals are unknown, limiting geochemical modeling of U transport in the White Canyon region.

Synthetic analogs of White Canyon uranyl sulfate minerals were crystallized through evaporation in isotemp ovens from solutions of U, S, and the appropriate cation. Each phase was characterized as follows: the crystal structures determined with single-crystal X-ray diffraction (SC-XRD), the bulk crystallinities verified with powder X-ray diffraction (PXRD), the cation concentrations measured with inductively coupled plasma optical emission spectroscopy (ICP-OES), and water content measured with thermogravimetric analysis (TGA). The ground samples were pressed into 5 mg pellets and dropped into an Alexsys high-temperature calorimeter to measure the enthalpy of solution, from which the enthalpy of formation (ΔH°_f ) was determined⁵. The structural complexity of each phase was calculated using the ToposPro software package^6,7.

This project has found that linear relationships may exist between the structural complexity and the ΔH°_f of these minerals, possibly suggesting the predominant formation mechanism of this mineral suite is energetic rather than entropic in nature. Continuing work will focus on determining the Gibbs free energies of formation (ΔG°_f) and entropies of formation (ΔS°_f ) of these uranyl sulfate phases, and determining relationships between the thermal properties and crystal complexity of these minerals.

References:

¹J. Plášil. (2014) J. Geosci. 59, 99-114.

²IMA Mineral List Database. Available online: http://rruff.info/ima/ (accessed 22 June 2019).

³Kampf, et al. (2018) Min. Mag. 83, 1-25.

⁴Ghurziy and Plášil (2019) Acta Cryst. B75, 39-48.

⁵A. Navrotsky. (2014) J. Am. Ceram. Soc. 97, 3349-3359.

⁶V.A. Blatov et al. (2014) Cryst. Growth Des. 14, 3576–3586.

⁷S.V. Krivovichev. (2016) Acta Cryst. B72, 274-276.