IMA 2018-J = redefinition and crystal chemistry of samarskite-(Y)

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IMA 2018-J = redefinition and crystal chemistry of samarskite-(Y)

Messaggio da Marco E. Ciriotti » mar 23 apr, 2019 13:56

Prossima pubblicazione.

▪ Britvin, S.N., Pekov, I.V., Krzhizhanovskaya, M.G., Agakhanov, A.A., Ternes, B., Schüller, W., Chukanov, N.V. (2019): Redefinition and crystal chemistry of samarskite-(Y), YFe3+Nb2O8: cation-ordered niobate structurally related to layered double tungstates. Physics and Chemistry of Minerals, 46, (in press).

Samarskite-(Y), the mineral known for almost 180 years only in metamict (X-ray amorphous) state, was found as non-metamict crystals in sanidinites of the Laach Lake (Laacher See), Eifel volcanic region, Germany. The crystal structure has been solved for the first time and refined to R1 = 1.1% based on 838 observed [I > 2σ(I)] independent reflections. Samarskite-(Y) is monoclinic, P2/c, a 9.8020(8), b 5.6248(3), c 5.2073(4) Å, β 93.406(4)°, V 286.59(4) Å3, Z = 2. The empirical formula (calculated on 8 O apfu) is [(Y0.25Ln0.18)Σ0.43Th0.37U0.134+Ca0.03]Σ0.96(Fe0.733+Mn0.182+)Σ0.91(Nb1.85Ti0.06Zr0.06Ta0.03W0.02)Σ2.02O8. Samarskite-(Y) from the type locality, the Blyumovskaya Pit, Ilmeny Mountains, South Urals, Russia, was studied for comparison; electron microprobe data showed the same species-defining constituents and stoichiometry:
[(Y0.34Ln0.20)Σ0.54U0.424+Th0.03]Σ0.99(Fe0.863+Mn0.082+)Σ0.94(Nb1.14Ta0.70Ti0.15)Σ1.99O8. Samarskite-(Y) is the first example of cation-ordered niobate structurally related to layered double tungstates AMW2O8, the compounds used as luminophors and active media in solid-state lasers. The pseudo-layered framework of the mineral can be derived from that of wolframite via substitution of W for Nb, whereas each second [FeO6] layer is replaced by [YO8] one. The resulting sequence of layers can be expressed as -[AO8]-[BO6]-[MO6]-[BO6]- leading to the formula AMB2O8 in which A = Y, Ln, Th, U4+, Ca; M = Fe3+, Mn2+; and B = Nb, Ta, Ti. The end-member formula of samarskte-(Y) is YFe3+Nb2O8 (approved by the Commission on New Minerals, Nomenclature and Classification, International Mineralogical Association, memorandum 90-FH/18).

NdR: il riferimento IMA chissà a cosa si riferisce… Quello corretto è nel titolo del thread.

Gruppo della samarskite

Q Calciosamarskite U4+CaNb2O8
Rd Ishikawaite U4+Fe2+Nb2O8
Rd Samarskite-(Y) YFe3+Nb2O8
Marco E. Ciriotti

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