[en] Bismuth calcium aluminate, Bi₂Ca₆Al₁₂O₂₇, has been prepared as a ceramic and a single crystal. Analysis of reciprocal space using both electron and X-ray diffraction show an R-centred hexagonal unit cell: a = b = 17.3892 (4), c = 6.986 (1) A. Additional weak reflections are observed; they require the introduction of a modulation wavevector q = 0.0453 (2)c* for indexing. The modulated structure has been solved using the superspace formalism [superspace group X anti 3(00γ)0]. A framework of corner-sharing AlO₄ tetrahedra forms corrugated sixfold rings and uncommon triple rings. The Ca²⁺ cations exhibit an eightfold coordination sphere; edge-sharing CaO₈ polyhedra form intertwinned zigzagging rows along c creating a three-dimensional net. Bi atoms are located in large hexagonal tunnels parallel to c and form Bi₂O₃ pairs, which adopt a trigonal bipyramidal configuration. The 6s² loneelectron pairs (Lp) point along c, in the opposite direction to the three Bi-O strong bonds to form two BiO₃Lp tetrahedra with a common base. Different orientations of the Bi₂O₃Lp₂ pairs, rotated by 60 around c, are observed. Their stacking modes in each of the hexagonal tunnels are described. The sequence of the stacking varies along c in each of the tunnels. (orig.)