Thermodynamics of Synthetic Zippeites

The uranyl mineral family includes the zippeite minerals, which are [UO2]+2 sulfates exhibiting well-known, well-described complex crystal structures. Zippeites have been widely observed in nature, yet they occur as fine grained powdery coatings in mixed aggregates which make their identification and characterization an arduous task. Despite being a minor species, zippeites serve as a model in the discovery of physical and chemical properties of other related minerals. Nevertheless, few studies exist regarding the thermodynamics of zippeites. Herein, I report on the synthesis of well-defined, pure zippeite powders and experimentally determined standard state enthalpy of formation values. Monovalent potassium and sodium zippeite K3(H2O)3.78[(UO2)4(SO4)2O3(OH)], Na5(H2O)11.16[(UO2)8(SO4)4O5(OH)3], and divalent magnesium zippeite, Mg(H2O)3.5[(UO2)2(SO4)O2] were prepared using mild hydrothermal synthesis. Characterization of the synthetic material was performed using XRD (X-ray powder diffraction), ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy), and TGA (Themorgravimetric Analysis). Standard state enthalpy of formation from the synthetic material for all three phases was obtained using high temperature oxide melt calorimetry. The stability of these phases is presented and discussed in the context of their crystal structures and related environmental implications.