| Summary: | 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 Sc<sup>3+</sup> and analyze its effect on the crystal structure of babingtonite. The Sc<sub>2</sub>O<sub>3</sub> 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 <i>Me</i><sup>2+</sup>:<i>Me</i><sup>3+</sup> = 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 <i>a</i> = 7.5272(1), <i>b</i> = 11.7175(1), <i>c</i> = 6.7613(1) Å, α = 91.710(1), β = 93.637(1), γ = 104.522(1)°, and <i>V</i> = 575.49(2) Å<sup>3</sup> for the Heftetjern specimen, and <i>a</i> = 7.5199(2), <i>b</i> = 11.7145(3), <i>c</i> = 6.7408(2) Å, α = 91.756(2), β = 93.786(2), γ = 104.549(2)°, and <i>V</i> = 573.83(3) Å<sup>3</sup> for the Baveno one. The structural formulae are <sup>A1</sup>Ca<sub>1.00</sub><sup>A2</sup>(Ca<sub>0.879</sub>Na<sub>0.121</sub>)<sup>M1</sup>(Sc<sup>3+</sup><sub>0.42</sub>Fe<sup>2+</sup><sub>0.37</sub>Mn<sup>2+</sup><sub>0.21</sub>)<sup>M2</sup>(Sc<sup>3+</sup><sub>0.68</sub>Fe<sup>2+</sup><sub>0.27</sub>Mg<sub>0.03</sub>Fe<sup>3+</sup><sub>0.02</sub>)Si<sub>5</sub>O<sub>14</sub>(OH) for Heftetjern, and <sup>A1</sup>Ca<sub>1.00</sub><sup>A2</sup>(Ca<sub>0.819</sub>Na<sub>0.181</sub>)<sup>M1</sup>(Sc<sup>3+</sup><sub>0.43</sub>Mn<sup>2+</sup><sub>0.36</sub>Fe<sup>2+</sup><sub>0.21</sub>)<sup>M2</sup>(Fe<sup>3+</sup><sub>0.36</sub>Fe<sup>2+</sup><sub>0.30</sub>Sc<sup>3+</sup><sub>0.26</sub>Sn<sup>4+</sup><sub>0.05</sub>Al<sub>0.03</sub>)Si<sub>5</sub>O<sub>14</sub>(OH) for Baveno. Due to Sc<sup>3+</sup> substitution, the <M2–O> distance, 2.09–2.11 Å, is longer than that of Sc-free babingtonite, 2.03–2.05 Å. The M2O<sub>6</sub> expansion leads to the lengthened O4–O10 edge shared between the M1O<sub>6</sub> and M2O<sub>6</sub> octahedra, and causes the stronger angular distortion of M2O<sub>6</sub>. 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.
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