Stability of U(VI) solid phases in the U(VI)-Ca2+-SiO2-OH system

Synthetic metaschoepite was altered at 70 C, for 30 days in silicate solutions of varying compositions to study alteration reactions and to compare experimental observations with a thermodynamic model of stability of the uranyl minerals metaschoepite ([(UO2)8O2(OH)12](H2O)10), becquerelite (Ca[(UO2)3O2(OH)3]2(H2O)8), α-uranophane (Ca[(UO2)(SiO3OH)]2(H2O)5) and soddyite ((UO2)2(SiO4).(H2O)2). Powder X-ray diffraction analysis, scanning electron microscopy and solution chemistry were used to monitor the alteration of the initial metaschoepite. In general, the secondary phases predicted by the thermodynamic model formed. However, an unexpected mineral (a calcium uranate) was observed above pH 8. Also, within the timeframe of these experiments, U(VI) silicate phases only formed at Si concentrations much higher than the lower field boundaries for uranophane and soddyite. (orig.)