The crystal structure of tunellite, SrB 6 O 9 (OH) 2 (H 2 O) 3 , monoclinic, a 14.415(3), b 8.213(1), c 9.951(2) Aa, beta 114.05(1) degrees , V 1075.8(4) Aa 3 , Z = 4, space group P2 1 /a, has been refined by full-matrix least-squares methods to an R index of 5.1% and a wR index of 3.7% for 2680 unique observed [F> or =5sigma (F)] reflections measured with MoKalpha X-radiation. The H positions were located on difference-Fourier maps and were refined using the "soft" constraint that H-O distances are approximately 0.96 Aa. The 6:(3Delta +3T) hexaborate fundamental building block (FBB) of the tunellite structure contains three (Bphi 4 ) tetrahedra (phi : unspecified ligand) and three (Bphi 3 ) triangles. All three (Bphi 4 ) tetrahedra share a central oxygen position, and the (Bphi 3 ) triangles each attach to two (Bphi 4 ) tetrahedra, forming three three-membered rings of the form (1Delta +2T). The FBBs polymerize to form sheets parallel to (100) by sharing four ligands with adjacent FBBs, and the irregular SrO 6 (H 2 O) 4 polyhedron occurs in voids within the heteropolyhedral sheet. Bonding between the sheets takes place via one Sr-OW-Sr bond and by a network of hydrogen bonds. There are eight hydrogen positions in the structure; five form bonds that bridge between adjacent heteropolyhedral sheets, two form bonds within the heteropolyhedral sheet, and one does not participate in a hydrogen bond.