Microporous titanosilicates - Synthesis and structural characterization of a new orthorhombic-type labuntsovite.

The crystal structure of a new orthorhombic-type labuntsovite with composition Na5.36Ti4(Si4O12)2(O0.34OH0.66)4·10.31H2O, 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 [Ti4(Si4O12)2(OH4-x Ox)](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.