Bornemanite is a rare alkali titanosilicate occurring in the natrolite zone of the Yubileynaya hyperagpaitic pegmatite, on Karnasurt Mountain, in the Lovozero massif, Kola Peninsula, Russia. The mineral is light yellow, lamellar (001) and elongate (010). No single crystals suitable for X-ray crystallography are available. New electron-microprobe chemical analyses, selected - area electron diffraction (SAED) and X-ray powder diffraction show that bornemanite, BaNa 3{(Na,Ti)4((Ti,Nb)2O2Si4O14) (F,OH)2}PO4, is monoclinic I11b, a 5.498(4), b 7.120(6), c 47.95(4) Å, � 88.4(1)°; Z = 4. By comparison with structural and chemical data for titanosilicates based on a bafertisite-like layer (heterophyllosilicates), a model of the structure of bornem anite has been obtained. This model has been refined by the distance least-squares technique (DLS program) and tested against calcu- lated powder-diffraction and SAED patterns. The structure of bornemanite can be described as a (001) stacking of heterophyllosilicate layers, where lomonosovite and seidozerite contents alternate in the interlayer spaces. Thus this structur e is the first documented case of a heterophyllosilicate based on modules of two other structures belonging to the same modular series, i.e., the mero-plesiotype bafertisite series. The lomonosovite-seidozerite polysomatic series is defined. In contrast to the original description, bornemanite is considered monoclinic and not orthorhombic, and lacks one cation per formula unit (mainly Na). Possible leaching of alkalis and the solid-state oriented transformation lomonosovite → bornemanite are discussed.