Microporous titanosilicates – Synthesis and structural characterization of a new orthorhombic-type labuntsovite MARCELLA CADONI and GIOVANNI FERRARIS* Dipartimento di Scienze Mineralogiche e Petrologiche, Universit` a di Torino, and Istituto di Geoscienze e Georisorse, CNR, Via Valperga Caluso, 35, 10125, Torino, Italy *Corresponding author, e-mail: giovanni.ferraris@unito.it Abstract: The crystal structure of a new orthorhombic-type labuntsovite with composition Na 5.36 Ti 4 (Si 4 O 12 ) 2 (O 0.34 OH 0.66 ) 4 ·10.31H 2 O, obtained as side product of hydrothermal runs devoted to the synthesis of rhodesite-type microporous silicates, has been solved and refined in the space group Cmmm (a = 7.278, b = 14.134, c = 7.118 Å; Z = 1) with X-ray single-crystal diffraction data collected by a Bruker-AXS Smart Apex diffractometer equipped with a CCD area detector. The crystal structure shows the overall features known for the heteropolyhedral framework (tetrahedral/octahedral) structures of the labuntsovite-group minerals, like systems of channels and disorder. The simpler chemical composition of the synthetic compound, which does not show isomorphous replacements, allows a detailed discussion of the zeolite-like disorder in the channels, which is related to the charge of the framework [Ti 4 (Si 4 O 12 ) 2 (OH 4-x O x )] (4+x)- and to the system of hydrogen bonding. The different space group Pbam shown by the two known natural labuntsovites (nenadkevichite and korobitsynite) is attributed to the presence in the natural samples of the Nb → Ti substitution. Key-words: labuntsovite group, zeolite-like structure, nenadkevichite, korobitsynite, microporous titanosilicates, hydrothermal synthesis. Introduction The labuntsovite group (Chukanov et al., 2002; 2003) in- cludes hydrous titanium and niobium alkali silicates repre- sented by the general formula A 4 B 4 [C 4-2x (H 2 O,OH) 2x ] D x [M 8 (Si 4 O 12 ) 4 (OH,O) 8 ]·nH 2 O. In this formula A and B rep- resent mainly alkalis; C and D represent several types of cat- ions, but the corresponding crystallographic sites exclude each other because the C – D distance is about 2 Å only; M represents mainly Ti and Nb. If the site D is occupied by cat- ions, to a maximum of x = 2, 2x oxygen atoms (belonging ei- ther to H 2 O molecules or OH groups) occur in C to complete the octahedral coordination of D. In the crystal structure, chains of corner-sharing M octahedra are linked by Si tetra- hedra to form a heteropolyhedral framework crossed by var- ious kinds of channels and cavities (Fig. 1). Consequently, the labuntsovite-group minerals show zeolitic-like proper- ties (Chukanov & Pekov, 2005) and belong to the growing family of micro- and mesoporous mineral phases (Ferraris & Merlino, 2005). Among the about thirty known members of the labuntsovi- te-group minerals (Chukanov et al., 2003), most are mono- clinic and only two members are orthorhombic: nenadkevi- chite (Perrault et al., 1973) and korobitsynite (Pekov et al., 1999; Niedermayr et al., 2002). In comparison to the ortho- rhombic members (a ~ 7.4, b ~ 14.2, c ~ 7.1 Å), the monoclin- ic members show a larger unit cell with a doubled a and, in some cases, also a doubled c (Chukanov et al., 2002); how- ever, being 117°, the latter direction cannot correspond to that of the orthorhombic c parameter. The chains of M oc- Fig. 1. Projection along [001] of the crystal structure of tsepinite-Ca to show the sites A, B, C, D within the channels of a monoclinic la- buntsovite. (Ti, Nb) octahedra are dotted. Eur. J. Mineral. 2007, 19, 217–222 DOI: 10.1127/0935-1221/2007/0019-1717 0935-1221/07/0019-1717 $ 2.70 2007 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart