The crystal structure and crystal chemistry of mendeleevite-(Ce), (Cs,□)6(□,Cs)_6(□,K)_6(REE,Ca,□)_(30)(Si_(70)O_(175))(H_2O,OH,F,□)_(35), a potential microporous material

摘要

The crystal structure of mendeleevite-(Ce), (Cs,□)6(□,Cs)_6(□,K)_6(REE,Ca,□)_(30)(Si_(70)O_(175))(H_2O,OH,F,□)_(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 R_1 = 4.15% based on 2274 observed [F_o > 4σ|F|] unique reflections measured with Mo-Kα, radiation on a Bruker PA diffractometer equipped with a CCD detector. Mendeleevite-(Ce) is cubic, space group Pm3-bar, a 21.9148(4) A, V 10525(1) A~3, Z = 2, D_(calc) = 3.066 g/cm~3. The empirical formula (electron microprobe)is Cs_(5.94)K_(2.22)[(Ce_(11.35)La_(5.86)Nd_(3.23)Pr_(1.54)Sm_(0.32)Gd_(0.20))_(Σ22.50)(Ca_(4.68)Sr_(1.00))_(Σ5.6 8)]_(Σ28.18)Si_(70.12)O_(203.17)H_(45.67)F_(6.83), Z=2, calculated on the basis of 210 (O + F) a.p.f.u., with H_2O and OH calculated from structure refinement (OH + F = 17 p.f.u.; H_2O = 17.75 p.f.u.).The structural formula is (Cs_(4.65)□_(1.35))_(Σ6)(□_(4.71)Cs_(1.29)_(Σ6)(□_(3.78)K_(2.22)_(Σ6){[Ce_(11.35)La_(5.86)Nd_(3.23) Pr_(1.54)Sm_(0.32)Gd_(0.20))_(Σ22.50)(Ca_(4.68)Sr_(1.00))_(Σ5.68)]_(Σ28.18□_(1.82)}_(Σ30)(Si_(70)O_(175))[(OH)_(10.17)F_(6.83)]_(Σ17)(H_2O)_(17.75). Simplified and endmember formulae are as follows: (Cs,□)_6(□,Cs)_6(□,K)_6(REE, Ca, □)_(30)(Si_(70)O_(175)) (H_2O,OH,F,□)_(35) and Cs_6(REE_(22)Ca_6)(Si_(70)O_(175))(OH,F)_(14)(H_2O)_(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 (Si_(104)O_(260))~(104-) framework and an (Si_(36)O_(90))~(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 H_2O 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.