Phosphosilicates of tetravalent uranium

A new variety of P-coffinite of (U,Ca,Fe)[(Si,P,S)O4] idealized formula where P and C are the mineral-forming elements equally to U and Si has been discovered. This allows one to enlarge the list of the known minerals of tetravalent uranium with a new mineral taxon of U4+ phosphosilicates. The crystalline structure of phosphosilicates is difficult to solve because of the dispersity of μm-sized crystals; therefore, crystallochemical analysis of probable transformations of mineral structures in polyhedrons was carried out. The consideration of the structures from coffinite (U4+-silicate) to ningyoite (Ca-U4+-phosphate) resulted in the conclusion on the possible existence of a series of U4+ minerals of mixed anionic and, hence, cationic composition, i.e., of phosphosilicates and silicophosphates of tetravalent uranium, with the indispensable presence of calcium in the crystalline structure.