Solid solution formation in the metatorbernite metazeunerite (Cu(UO2)2(PO4)2-x(AsO4)x.nH2O) and their stability under conditions of variable temperature

Mineral phases which can be thought of as members of a metatorbernite–metazeunerite solid solution (Cu(UO2)2(PO4)2−x(AsO4)x.8H2O have been identified in radioactive samples from spoil heaps at the uranium mine site in South Terras, Cornwall (grid reference SW935523). A complete solid solution (0 < x < 2) was synthesized by precipitation from solution using uranium (VI) nitrate and copper (II) chloride and phosphoric acid/arsenic acid in the appropriate molar proportions. Refined unit cell parameters determined by Pawley fitting of powder X-ray diffraction data showed a linear variation in the a unit cell parameter according to Vegard's Law, allowing the composition of the natural mineral phases found at South Terras to be determined from measurement of their unit cell parameters. High-resolution variable-temperature synchrotron powder X-ray diffraction studies were carried out at the Diamond Light Source on three members of this solid solution (x = 0, 1, 2) and showed different structural behaviour as a function of composition and temperature. Metatorbenite (x = 0) retains its tetragonal symmetry at low temperatures and dehydrates to an amorphous phase at 473 K, whereas metazeunrite (x = 2) transforms to an orthorhombic phase at low temperatures, regains its tetragonal symmetry on heating to 323 K and undergoes a further transition to an, as yet, unidentified phase at 473 K.

This article is part of the theme issue ‘Fifty years of synchrotron science: achievements and opportunities’.