Stabilisation of divalent rare earth elements in natural fluorite

Summary

¶The occurrence of divalent rare earth elements (Sm²⁺, Yb²⁺, Tm²⁺, and Ho²⁺) in natural fluorite is evaluated using a suite of 37 samples deriving mainly from Sn–W deposits in the Erzgebirge (Germany), Central Kazakhstan, and the Mongolian Altai. Trace element composition was determined by ICP-AES and ICP-MS. The defect structure of the samples was studied by cathodoluminescence (CL), electron paramagnetic resonance (EPR), and optical absorption spectroscopy. Reduction of cubic Sm³⁺, Yb³⁺, Tm³⁺, and Ho³⁺ under radioactive irradiation produces the corresponding divalent centres. Our data suggest a preferable formation of Sm²⁺ and Yb²⁺ under thorium and of Tm²⁺ and Ho²⁺ under uranium irradiation. Irradiation (indicated by intense brownish (thorium) and deep purple (uranium) coloration of fluorite) gives rise to a population of divalent centres in equilibrium with their decay. However, sporadic radioactive irradiation and stabilisation of the divalent state of the REE by other electron defects were found in most cases.

Three models of stabilisation of Sm²⁺, Yb²⁺, Tm²⁺, and Ho²⁺ are discussed. The most effective mechanism for Sm, Yb, Tm, and Ho is coupling with Fe³⁺ centres (REE³⁺+Fe²⁺ → REE²⁺+Fe³⁺). Accordingly, the occurrence of Fe³⁺ centres in natural fluorite is regarded to indicate not an oxidising, but rather a reducing environment during fluorite precipitation. Originally incorporated in the divalent form, Fe²⁺ was converted to Fe³⁺ by radioactive irradiation. Such a conclusion is in agreement with the finding of high contents of interstitial fluorine providing tetragonal local compensation of trivalent REE centres in crystals with high Fe³⁺. If Fe is not present, compensation of divalent Sm, Yb, and Tm is achieved by radiogenic oxidation of Ce(Pr, Tb)³⁺ accompanied by charge transfer (REE³⁺+Ce(Pr, Tb)³⁺ → REE²⁺+ Ce(Pr, Tb)⁴⁺). Ho²⁺ is sometimes stabilised by a hole trapped by an electron localised on a F vacancy (Ho³⁺+e⁻ on □_F → REE²⁺+ self-trapped exciton). Because Sm²⁺ is optically active, the stabilisation by Fe³⁺ (stable up to temperatures above 350 °C) or Ce(Pr, Tb)⁴⁺ (unstable even under visible light) in samples may be determined by careful observations in the field.