Crystal structure of eveslogite revealed by electron diffraction techniques: a titanosilicate mineral with structurally and compositionally distinct nanoscopic tubular units.

While silicate nanotubules are unknown in synthetic materials so far, naturally occurring minerals provide several examples of nanoscopic tubular structural building units based upon silicate oxyanions. The crystal structure of eveslogite, a chemically and structurally complex natural silicate mineral from Eveslogchorr mountain, Khibiny massif, Kola Peninsula, Russia, with an idealized sum formula of K17.5(Ba,Sr)4(Na,Ca)40[(Ti,Nb,Fe,Mn)11Si62O179(OH,F)12(O,OH)13](H2O) has been solved using modern electron crystallography techniques. This study reveals the first example of a binary structure formed by two different types of transition-metal-modified silicate nanotubular units in a fibrous crystal. One of these is closely related to the nanorod occurring in the unitary nanotubule-based structure of yuksporite, whereas the other is unprecedented and can be considered as an (Nb, Ti)-modified derivative of the [Si12O30]12- nanotubular units occurring in the binary all-silicate nanotubule-based structures of charoite polytypes. The eveslogite tubular units are packed in a tight arrangement and linked with each other through secondary interactions involving Ca2+- and Na+-centered polyhedra. The interiors and walls of the tubules are occupied by K+ and Ba2+ cations as well as H2O molecules. Topological analysis of the interpolyhedral connectivity shows that the walls of the nanotubules possess topologies related to those of lamprophyllite and delhayelite-group minerals. This topological relationship may indicate possible structural pathways linking these framework types and highlights interesting similarities of exfoliation processes that are well known in modern soft chemistry nanotechnologies. Information-based structural complexity calculations place eveslogite among the most complex minerals known so far. Our study further demonstrates the enormous potential of mineralogy to discover structures unprecedented among the currently known synthetic materials and may serve as an inspiration for the preparation of novel types of artificial nanostructures and their possible technical applications in various fields.