The crystal structure of Al4B2O9, synthesized from Al(NO)(3)center dot 9H(2)O and B(OH)(3) via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with (11) B and Al-27 MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al32B16O72 is closely related to the boralsilite (Al32B12Si4O74) Structure with Si replaced by B and to mullite (Al4+2xSi2-2xO10-x) It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) angstrom, b = 5.5413(2) angstrom, c = 15.0531(6) angstrom, beta = 90.913(2)degrees, Z = 8 for Al4B2O9. The main structural units are isolated chains of edge-sharing AlO6-octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO4, AlO5, BO3, and BO4 groups with two B atoms and one O atom (O5') disordered on interstitial positions. Al-27 and B-11 NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold: [fourfold + fivefold] =similar to 50%:50%) and of threefold and fourfold coordinated B (similar to 80%:20%).