The Role of Scandium Substitution in Babingtonite Group Minerals

Abstract

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) Å3 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) Å3 for the Baveno one. The structural formulae are A1Ca1.00A2(Ca0.879Na0.121)M1(Sc3+0.42Fe2+0.37Mn2+0.21)M2(Sc3+0.68Fe2+0.27Mg0.03Fe3+0.02)Si5O14(OH) for Heftetjern, and A1Ca1.00A2(Ca0.819Na0.181)M1(Sc3+0.43Mn2+0.36Fe2+0.21)M2(Fe3+0.36Fe2+0.30Sc3+0.26Sn4+0.05Al0.03)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.