THE STRUCTURAL STABILITY OF THE ZIPPEITE GROUP OF URANYL SULFATE MINERALS COMMON IN MINE WASTES

Zippeite group minerals are uranyl sulfate minerals that form on uranium mine wastes. The zippeite structure contains sheets of uranyl pentagonal bypiramids and sulfate tetrahedra and these sheets are linked through bonds to monovalent and divalent cations, as well as through hydrogen bonds associated with water groups. They form by alteration of other uranium minerals or when there is significant evaporation of water containing sulfate and uranium. Despite the importance of zippeite group minerals in impacting uranium concentrations in water and the mobility of uranium at mine waste sites, the thermodynamic properties that govern their formation and structural stability in different environments are poorly constrained.

In this study, pure monovalent (Na, K) and divalent (Co, Zn, Mg) zippeite were synthesized hydrothermally. Using high temperature oxide melt solution calorimetry, their standard-state enthalpy of formation from the elements and from their binary oxides was measured in order to investigate relations between their crystal chemistry and their structural stability. Our results show that there is a linear trend between the stability of these structures and the ionic radius of their interlayer cations. Also, there is a positive correlation between the enthalpy of formation from the oxides and the basicity of oxides related to the interlayer cations. Our results allow prediction of the stability of zippeite group minerals relative to other uranyl minerals as well as the structural stability of zippeites with interlayer cations other than those mentioned in this study.