High-Temperature Crystal Chemistry of Meta-Autunite Group Minerals: Metatorbernite, Cu[sub.8]

Thermal expansion of metatorbernite, Cu(UO[sub.2] )[sub.2] (PO[sub.4] )[sub.2] (H[sub.2] O)[sub.8] (1), and metazeunerite, Cu(UO[sub.2] )[sub.2] (AsO[sub.4] )[sub.2] (H[sub.2] O)[sub.8] (2), 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, 1 is stable up to 300(50) K, while 2 is stable up to 250(50) K. Powder XRD studies at various temperatures suggest that 1 dehydrates in three stages at ca. 353, 373, and 483 K, while 2 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 PO[sub.4] and AsO[sub.4] tetrahedra.