A novel approach to the structural distortions of U/Th snub-disphenoids and their control on zircon -> reidite type phase transitions of U1-xThxSiO4

摘要

Coffinite (USiO4) and thorite (ThSiO4) are conspicuous radiogenic silicates in the geonomy. They form U1-xThxSiO4 (uranothorite) solid solutions in zircon-type phase. Investigating the phase-evolution of these minerals is of utmost significance in realizing their applicability in the front-as well as at the back-end of nuclear industries. We carried out a systematic study of zircon- to reidite-type (tetragonal I4(1)/amd to I4(1)/a) structural transitions of U1-xThxSiO4 solid solution, and investigated their mechanical behaviour. We found a unique behaviour of transition pressure with the change in U-Th concentration in the solid solution. The phase transition pressure (p (t)) is found to be minimum for x = 0.5. We develop the necessary formalism and present an efficient method to estimate the longitudinal and angular distortions of U/ThO8-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: delta (longitudinal distortions) and sigma(2) (angular distortions) to quantify the polyhedral distortions. A detailed analysis of U/ThO8 snub-disphenoidal distortions is presented to address such variation of p (t) with U and Th concentration. We argue that our approach is independent of polyhedral volume and can be used for any AB(8) (A: cation, B: anion) type snub-disphenoidal system.