For the first time large single-crystals of taranakite K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 18H 2 O were grown by gel crystallization. An X-ray structure determination showed taranakite to be the mineral with the longest crystallographic axis described hitherto. X-ray crystal data are: space group R3c, a = 870.25(11), c = 9505(1) pm, Z = 6, R g = 0.028. A neutron scattering experiment with subsequent Rietveld refinement of the powder pattern of deuterated taranakite elucidated the D-atom positions. Neutron crystal data are: a = 868.82(3), c = 9498(2) pm, R w p = 0.061. Taranakite is a layer structure mineral having six layers of composition [K 3 Al 5 (HPO 4 ) 6 (PO 4 ) 2 (H 2 O) 1 2 ] separated from each other by water layers. The rigid layer is formed by columns of corner sharing hydrogen phosphate tetrahedra and Al octahedra which are interconnected by additional six-coordinated Al ions. In trigonal holes of the layer orthophosphate ions are situated. Non-exchangeable K ions are trapped within the layers. Hydrogen bonds in taranakite are formed between building units within the rigid layer, within the water interlayer, and between layer and interlayer. Similarities and differences between the coordination of interlayer water in taranakite and ice I h are discussed. 3 1 P solidstate MAS NMR spectra show two isotropic lines at δ i s o = -17.2 and 5.2 ppm. The intensity ratio 3:1 of the two sideband systems corresponds to the molar ratio of HPO 4 2 - and PO 4 3 - units in the structure of taranakite. The P(1) atom of hydrogen phosphate has a larger anisotropy in agreement with the more different P-O bond lengths of HPO 4 2 - compared to PO 4 3 - . The 3 1 P nuclear magnetic shielding tensor of HPO 4 2 - is characterized by larger shielding values σ 2 2 and σ 3 3 .