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Replacement textures involving four scandium silicate minerals in the Heftetjern granitic pegmatite, Norway

¹ Geologisk Museum, Universitetet i Oslo, Postboks 1172, Blindern, N-0318 Oslo, Norway
² Institut für Mineralogie und Petrologie, Karl-Franzens-Universität Graz, Universitätsplatz 2, A-8010 Graz, Austria
³ CNR - Istituto di Geoscienze e Georisorse (IGG), Sezione di Pavia, Via Ferrata, 1, 1-27100 Pavia, Italy

^* Corresponding author, e-mail: gunnar.raade{at}nhm.uio.no

Four late-hydrothermal scandium silicate minerals, related by replacement textures, were studied by EMPA and SIMS. Textural evidence shows that early-formed thortveitite is broken up by bazzite and scandian milarite, which seem to occur in equilibrium. Thortveitite is also partly replaced by kristiansenite. The two steps of alteration involve first the introduction of fluids rich in Be, K, Ca and Cs to form bazzite and milarite and then Ca and Sn for the formation of kristiansenite. Addition of water accompanies both steps. Thortveitite is unusually rich in SnO₂ (up to 5.67 wt.%). The amount of Sn is balanced by Mn according to the substitution scheme 2Sc³⁺ Sn⁴⁺ + Mn²⁺. Bazzite is strongly zoned with respect to Cs and has up to 8.55 wt% Cs₂O. The alkali content in the structural channels is balanced by Fe²⁺ and Mn²⁺ substituting for Sc. Scandian milarite is close to the end-member formula K(CaSc)Be₃(Si₁₂O₃₀) with nearly all Al replaced by Be and half of the Ca atoms by Sc. Kristiansenite is very near to the ideal formula Ca₂ScSn(Si₂O₇)(Si₂O₆OH).

Key-words: thortveitite, kristiansenite, scandian milarite, cesian bazzite, chemical analysis, Norway.

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