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).