High-Temperature Crystal Chemistry of Meta-Autunite Group Minerals: Metatorbernite, Cu(UO2)2(PO4)2(H2O)8 and Metazeunerite, Cu(UO2)2(AsO4)2(H2O)8

Thermal expansion of metatorbernite, Cu(UO2)(2)(PO4)(2)(H2O)(8) (<bold>1</bold>), and metazeunerite, Cu(UO2)(2)(AsO4)(2)(H2O)(8) (<bold>2</bold>), has been investigated using single-crystal and powder X-ray diffraction. Both minerals are prone to dehydration, which proceeds already at ambient conditions. According to the single-crystal XRD data, <bold>1</bold> is stable up to 300(50) K, while <bold>2</bold> is stable up to 250(50) K. Powder XRD studies at various temperatures suggest that <bold>1</bold> dehydrates in three stages at ca. 353, 373, and 483 K, while <bold>2</bold> in two stages at ca. 283 and 543 K. Calculation of the main coefficients of thermal expansion reveals strong anisotropy. The expansion is maximal in the direction normal to the autunite-type layers. This correlates with the anisotropy in thermal evolution of Cu-O bond lengths and differences in the thermal behavior of PO4 and AsO4 tetrahedra.