Kanemite was studied by single-crystal X-ray diffraction. The mineral, ideally NaHSi 2 O 5 .3H 2 O, is orthorhombic (space group Pbcn); unit-cell parameters are a = 4.946(3), b = 20.502(15), c = 7.275(3) Aa, with Z = 4. The structure is solved and refined to an R value of 0.058 for 825 independent reflections. The arrangement of atoms consists of alternating (010) sheets of corrugated [Si 2 O 4 OH] (super n-) n and hydrated Na. The silicate sheets contain six-membered rings of HOSiO 3 -SiO 4 units. Sodium atoms coordinate to six water molecules, forming layers of distorted octahedra. Residual electron densities were located that give reasonable positions for four H atoms. One H is part of a silanol group, and the other three H atoms are associated with water bonded to Na. Bonding between the silicate and Na sheets is through hydrogen bonding from H of the Na layer to O of the silicate sheet.