The crystal structure of kaliborite, KMg 2 H[B 6 O 8 (OH) 5 ] 2 (H 2 O) 4 , monoclinic, a 18.572(6), b 8.466(3), c 14.689(5) Aa, alpha 100.02(3) degrees , V 2274(1) A degrees 3 , Z = 4, space group C2/c, has been refined by full-matrix least-squares methods to an R index of 4.4% and a wR index of 4.7% for 2403 unique observed [F> or =5sigma (F)] reflections measured with MoKalpha X-radiation. The kaliborite structure contains a 6:(2Delta +3T)+Delta fundamental building block (FBB). Each FBB contains a pentaborate (2Delta +3T) unit composed of two three-membered (1Delta +2T) rings and an additional triangle that is attached to the pentaborate unit. The borate FBBs polymerize, forming zigzag chains along b. The Mg cation is in octahedral coordination, and linked to an adjacent borate chain. The heteropolyhedral chains are joined through the K atoms, which are in a distorted cubic coordination, and via hydrogen bonds. Nine of the ten hydrogen atoms provide bonding between the heteropolyhedral chains, the other hydrogen atom links anions of the same heteropolyhedral chain. Kaliborite is one of two borate minerals that are known to contain a crystallographically symmetrical hydrogen bond.