Centric or acentric crystal structure for natural schmitterite, UTeO5? New evidence from a crystal from the type locality

Summary Schmitterite, UTeO₅, was found in a sample of the mineralogical collection of the Natural History Museum of the University of Florence. The sample is from the Bambolla mine, Moctezuma, Sonora, Mexico and the mineral occurs as very rare crystals associated with emmonsite. Schmitterite is orthorhombic, space group Pca2₁, with the following unit-cell parameters: a = 10.163(1), b = 5.361(1), c = 7.865(1) ?, V = 428.5(1) ?³, and Z = 4. EDS chemical analyses showed only uranium and tellurium to be present. The crystal structure has been solved and refined to R = 2.28%. It consists of edge-sharing chains of UO₇ pentagonal bipyramids parallel to 001] linked into a framework by TeO₄ polyhedra. The crystal-chemical reasons for the choice of the non-centrosymmetric arrangement and the structural relationships with other natural tellurites are outlined.