Abstract: Frankamenite, K3Na3Ca5[Si12O30]F3(OH)1H2O, from the Yakutian charoitic rocks, has a triclinic unit cell with the dimensions: a = 10.094(3), b = 12.691(3), c = 7.240(1) Å, α = 90.00(2)°, β = 111.02(2)°, γ = 110.20(2)°, sp.gr. = P1, Z = 1. Its crystal structure has been refined to Raniso= 0.059 (Rw = 0.060) using the intensities of 2563 reflections.
The bases of the structure are zig-zag walls of Ca, Na octahedra joined by infinite octagonal cross-sectioned tubes of SiO4 of the composition (Si12O30). Unlike canasite from the Khibini Mountains, the triclinic cell of frankamenite contains only one silicate tube. The atomic coordinates, interatomic distances and angles are topologically similar to those of canasite. Major differences are observed in the distribution of octahedral cations, which reflect the different anion compositions. Frankamenite exhibits six mixed Ca-Na positions, whereas canasite has only one such position. The H2O molecule is located in the centre of the silicate tube between K+ cations.