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- Skoda, Radek, et al.
(författare)
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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
- 2015
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 79:4, s. 965-983
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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