The crystal structure of mendeleevite-(Ce), (Cs,☐)6(☐,Cs)6(☐,K)6(REE,Ca,☐)6(Si70O175) (H2O,OH,F,D)35, a new mineral from the moraine of the Darai-Pioz glacier, the Alai mountain ridge. Tien-Shan mountains, northern Tajikistan, was solved by direct methods and refined to Ri = 4.15% based on 2274 observed [Fo > 4σ|F|] unique reflections measured with Mo-Kα. radiation on a Bruker PA diffractorneter equipped with a CCD detector. Mendeleevite-(Ce) is cubic, space group Pm3̄, a 21.9148(4) Å, V 10525(1) Å3, Z = 2, Dcalc = 3.066 g/cm3. The empirical formula (electron microprobe) is CS5.94K2.22[(Ce11.35La5.86Nd3.23Pr1.54Sm0.32Gd0.20)Σ22.50(Ca4.68Sr1.00)Σ5.68]Σ28.18 Si70.12O203.17H45.67F6.83, Z = 2, calculated on the basis of 210 (O + F) a.p.f.u., with H2O and OH calculated from structure refinement (OH + F = 17 p.f.u.; H2O = 17.75 p.f.u.).

The structural formula is (Cs4.65☐1.35)Σ6(☐4.71Cs1.29)Σ6(☐3.78K2.22)Σ6﹛[(Ce11.35La5.86Nd3.23 Pr1.54Sm0.32Gd0.20)Σ22.50(Ca4.68Sr1.00)Σ5.68]Σ28.18☐1.82﹜Σ30(Si70O175)[(OH)10.17F6.83]Σ17(H2O)17.75. Simplified and endmember formulae are as follows: (Cs,☐)6(☐,Cs)6(☐,K)6(REE,Ca,☐)30(Si70O175) (H2O,OH,F,☐)35 and Cs6(REE22Ca6)(Si70O175)(OH,F)14(H20)21. The crystal structure of mendeleevite-(Ce) is an intercalation of two independent Si—O radicals and an M framework of (REE,Ca) polyhedra. The Si—O radicals are an (Si140O260)104– framework and an (Si36O90)36– cluster which do not link directly. The M framework is located between the Si—O framework and the Si—O clusters. Interstitial cations occupy two types of cages and channels. Cages I and II are 78 and 22% occupied by Cs. Channels along [100↻] contain K atoms and H2O groups. Mendeleevite-(Ce) has no natural or synthetic structural analogues. Mendeleevite-(Ce) is a framework mineral with large cavities and it has the potential to be used as a model for the synthesis of microporous materials of industrial interest.