The crystal structure of seidite-(Ce) was studied on the material from the holotype deposit (Yubileynaya pegmatite, Lovozero alkaline massif, Kola Peninsula, Russia) using CuKα radiation. The studied crystal of seidite-(Ce) has a monoclinic symmetry, P21/m, a = 7.2416(7) Å, b = 24.218(5) Å, c = 14.3760(8) Å, β = 90.049(8)ο, V = 2521.3(6) Å3. The crystal structure was solved on a single crystal by direct methods and refined to R1 = 0.30 on the basis of 2982 independent reflections with I > 2σI. The model obtained by the single-crystal method was confirmed by the Rietveld method (Rwp = 0.016, RBragg = 0.011 for 3197 reflections). The model of the crystal structure of the mineral differs from the one proposed earlier. It is based upon the triple (SrCe)(TiO)[Si8O20(OH)2F]4– layers parallel to (010) and consisting of two mirror-symmetric silicon-oxygen Si4O10(OH) layers formed by ten-membered tetrahedral rings with the ratio of Q2 : Q3 = 2 : 3. The layers are stacked together through TiO5F octahedra and chains of face-sharing (Ce,Sr)O8 bicapped trigonal prisms parallel to the c axis. The Na and K atoms, alongside H2O molecules, occupy the intra- and interlayer spaces of the structure. The crystal chemical formula of the mineral can be written as (Na3.15K0.30Ca0.10)(Ce1.38Sr0.62)(TiO)Si8O20(OH)2F ⋅ H2O, and the idealized end-member formula as Na4(SrCe)(TiO)Si8O20(OH)2F ⋅ H2O. The data obtained suggest the existence of two or more polytypes of seidite-(Ce), as well as the structural relationship between the mineral and laplandite-(Ce).
Panikorovskii et al. (2026) studied this question.