The aqueous chemistry of uranium minerals. 4. Schröckingerite, grimselite, and related alkali uranyl carbonates

Abstract

AbstractUsing solution techniques, ΔG°f(298.2k) has been calculated for schröckingerite, NaCa3UO2(CO3)3 SO4F · 10H2O, and grimselite, NaK3UO2(CO3)3 · H2O. The values are −8077.3 ±8.7 and −4051.3 ±1.8 kJ mol−1 respectively. ΔH°f(298.2k) for grimselite is −4359.0 ± 1.8 kJ mol−1. An estimate for ΔG°f of the synthetic compound Na4UO2(CO3)3(s) using previously published equilibria has been made, −3720.0±9.7 kJ mol−1. The results have been used to reconstruct the chemical environments in which these and related compounds can form. Grimselite is only stable at relatively high activities of K(aq)+ and the alkali metal carbonates containing only potassium or sodium ions have not been reported to occur naturally. This is most probably due to their relatively high solubility in water. Schröckingerite can form from solutions which are more than saturated with respect to calcite and fluorite. The close relationship of schröckingerite with gypsum, observed frequently in the field, is evident from synthetic and solution studies.