The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO 4 (M = Ce, U)

Orthosilicates adopt the zircon structure types ( 4 ), consisting of isolated SiO tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development, and material science. Stetindite (CeSiO ) and coffinite (USiO ) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, high-temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 to ∼850 °C. Stetindite maintains its 4 symmetry with increasing temperature but exhibits a discontinuous expansion along the axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the -axis. Additional high-temperature Raman and Fourier transform infrared spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C and then thermally decompose into a mixture of UO and SiO . A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high-temperature structures of two coffinite-thorite solid solutions, uranothorite (U Th SiO ), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO , USiO , U Th SiO , and U Th SiO were determined to be α = 14.49 × 10 , 14.29 × 10 , 17.21 × 10 , and 17.23 × 10 °C , respectively.