The Role of Scandium Substitution in Babingtonite Group Minerals

Sc-rich babingtonite from Heftetjern, Norway and Baveno, Italy were examined using electron microprobe analysis and X-ray single-crystal refinement in order to re-examine the behavior of Sc³⁺ and analyze its effect on the crystal structure of babingtonite. The Sc₂O₃ content is 13.78 wt.% in the Heftetjern specimen, and 8.44 wt.% in the Baveno one. In contrast, the latter has higher Fe content (11.13 wt.% as FeO) rather than the former one (8.63 wt.% as FeO). Characteristically, both specimens contain sodium. Although the oxidation state of octahedral cations in babingtonites is in general Me²⁺:Me³⁺ = 1:1, trivalent cations in the Heftetjern specimen attain 1.14 apfu. This excess of trivalent cations must be counterbalanced by monovalent Na substituted for Ca. The unit-cell parameters are a = 7.5272(1), b = 11.7175(1), c = 6.7613(1) Å, α = 91.710(1), β = 93.637(1), γ = 104.522(1)°, and V = 575.49(2) ų for the Heftetjern specimen, and a = 7.5199(2), b = 11.7145(3), c = 6.7408(2) Å, α = 91.756(2), β = 93.786(2), γ = 104.549(2)°, and V = 573.83(3) ų for the Baveno one. The structural formulae are ^A1Ca_1.00^A2(Ca_0.879Na_0.121)^M1(Sc³⁺_0.42Fe²⁺_0.37Mn²⁺_0.21)^M2(Sc³⁺_0.68Fe²⁺_0.27Mg_0.03Fe³⁺_0.02)Si₅O₁₄(OH) for Heftetjern, and ^A1Ca_1.00^A2(Ca_0.819Na_0.181)^M1(Sc³⁺_0.43Mn²⁺_0.36Fe²⁺_0.21)^M2(Fe³⁺_0.36Fe²⁺_0.30Sc³⁺_0.26Sn⁴⁺_0.05Al_0.03)Si₅O₁₄(OH) for Baveno. Due to Sc³⁺ substitution, the <M2–O> distance, 2.09–2.11 Å, is longer than that of Sc-free babingtonite, 2.03–2.05 Å. The M2O₆ expansion leads to the lengthened O4–O10 edge shared between the M1O₆ and M2O₆ octahedra, and causes the stronger angular distortion of M2O₆. This can be explained by the increase of the O4–M1–O10 angle and decrease of the O4–M1–O8 angle with lengthening of the O4–O10 edge.