Redefinition and crystal chemistry of samarskite-(Y), YFe³⁺Nb₂O₈: cation-ordered niobate structurally related to layered double tungstates

Abstract

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) ų, Z = 2. The empirical formula (calculated on 8 O apfu) is $\left[ {\left( {{\text{Y}}_{0. 2 5} Ln_{0. 1 8} } \right)_{\varSigma 0. 4 3} {\text{Th}}_{0. 3 7} {\text{U}}_{0. 1 3}^{ 4+ } {\text{Ca}}_{0.0 3} } \right]_{\varSigma 0. 9 6} \left( {{\text{Fe}}_{0. 7 3}^{ 3+ } {\text{Mn}}_{0. 1 8}^{ 2+ } } \right)_{\varSigma 0. 9 1} \left( {{\text{Nb}}_{ 1. 8 5} {\text{Ti}}_{0.0 6} {\text{Zr}}_{0.0 6} {\text{Ta}}_{0.0 3} {\text{W}}_{0.0 2} } \right)_{\varSigma 2.0 2} {\text{O}}_{ 8}$. 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: $\left[ {\left( {{\text{Y}}_{0. 3 4} Ln_{0. 20} } \right)_{\varSigma 0. 5 4} {\text{U}}_{0. 4 2}^{ 4+ } {\text{Th}}_{0.0 3} } \right]_{\varSigma 0. 9 9} \left( {{\text{Fe}}_{0. 8 6}^{ 3+ } {\text{Mn}}_{0.0 8}^{ 2+ } } \right)_{\varSigma 0. 9 4} \left( {{\text{Nb}}_{ 1. 1 4} {\text{Ta}}_{0. 70} {\text{Ti}}_{0. 1 5} } \right)_{\varSigma 1. 9 9} {\text{O}}_{ 8}$. 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).