Lüneburgite, Mg3(H2O)6[B2(OH)6(PO4)2], is triclinic, P1, with a = 6.3475(6), b = 9.8027(11), c = 6.2976(5) Å, α = 84.46(1), β = 106.40(1), γ = 96.40(1)° and Z = 1. Its structure has been refined to an unweighted residual of 0.026 using 2069 reflections. The structure consists of T2M-M-T2M sandwiches stacked along b* with adjacent sandwiches only connected by H bonds. The fundamental cluster in the T2M layers is a Mg(2) octa-hedron connected to a B-P tetrahedral pair. All the tetrahedra in a T2M layer have their apices pointing in the same direction. The apices of the tetrahedra in the upper T2M layer in a T2M-M-T2M unit point down into the interior of the unit, and the apices of the tetrahedra of the lower T2M layer point up into the interior of the unit. The upper and lower T2M layers are bridged by Mg(1) octahedra. Extensive H bonding exists within T2M-M-T2M units as well as between them. Two small intersecting channels run parallel to a and c within each T2M-M-T2M unit. The diameter of each channel, excluding H bonds, is approximately 4 Å.