Redefinition of thalenite-(Y) and discreditation of fluorthalenite-(Y): A re-investigation of type material from the Osterby pegmatite, Dalarna, Sweden, and from additional localities

Using type material from the Osterby pegmatite in Dalarna, Sweden, the chemical composition and structural parameters of thalenite-(Y) [ideally Y3Si3O10(OH)] were examined by wavelength dispersive spectroscopy electron microprobe (WDS EMP) analysis and single-crystal X-ray diffraction. High contrast back-scatter electron images of the Osterby material show at least two generations of thalenite-(Y). The formula of the primary thalenite-(Y) normalized to 11 anions is (Y2.58Dy0.11Yb0.09Gd0.06Er0.06Ho0.02 Sm0.02Tb0.02Lu0.02Nd0.01Tm0.01)(Sigma 3.00)Si3.01O10F0.97OH0.03. The secondary thalenite-(Y), replacing the primary material, is weakly enhanced in Y and depleted in the lightest and the heaviest rare-earth elements, yielding the formula (Y2.63Dy0.12Yb0.06Gd0.06Er0.05Ho0.02Sm0.02Tb0.02Tm0.01Nd0.01Lu0.01)(Sigma 3.00) Si3.01O10F0.98OH0.02. Structural data for thalenite-(Y) from Osterby clearly indicate the monoclinic space group P2(1)/n, with a = 7.3464(4), b = 11.1726(5), c = 10.4180(5) angstrom, beta = 97.318(4)degrees, V = 848.13(7) angstrom(3), Z = 4, which is consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R-1 = 0.0371 for 1503 unique observed reflections, and the final chemical composition obtained from the refinement, (Y2.64Dy0.36)(Sigma 3.00)F-0.987[Si3O10], Z = 4, is in good agreement with the empirical formula resulting from electron microprobe (EMP) analysis. Both techniques reveal a strong dominance of F over OH, which means that the type material actually corresponds to the fluorine analogue. Moreover, new EMP analyses of samples of thalenite-(Y) from an additional seven localities (angstrom skagen and Reunavare in Sweden; White Cloud and Snow Flake in Colorado, USA; the Guy Hazel claim in Arizona, USA; Suishoyama and Souri in Japan) clearly show the prevalence of F over OH as well. Based on these observations, the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association has recommended a redefinition of the chemical composition of thalenite-(Y) to represent the F-dominant species with the ideal formula Y3Si3O10F, as it has historical priority. Consequently, the later described fluorthalenite-(Y) has to be discredited.