The crystal structure of zigrasite, ideally MgZr(PO4)2(H2O)4, a 5.3049(2), b 9.3372(4), c 9.6282(5) Å, α 97.348(1)°, β 91.534(1)°, γ 90.512(4)°, V 472.79(5) Å3, Z = 2, triclinic, P1, Dcalc 2.66 g.cm−3, from the giant 1972 pocket at Newry, Oxford County, Maine, USA, has been solved and refined to R1 3.75% on the basis of 2623 unique reflections (Fo >4σF). There are two P sites, each of which is solely occupied by P with <P–O> distances of 1.532 and 1.533 Å, respectively. There are two Mg sites, both of which are occupied by Mg and are octahedrally coordinated two O anions and four (H2O) groups with <Mg–O> distances of 2.064 and 2.075 Å, respectively. There is one Zr site, occupied by Zr and octahedrally coordinated by six O anions with a <Zr–O> distance of 2.065 Å. The (ZrO6) octahedron shares corners with six (PO4) tetrahedra, forming a [Zr(PO4)2] sheet parallel to (001). These sheets are stacked in the c direction and linked by (MgO2(H2O)4) octahedra that share O atoms with the (PO4) groups. The structure is formally a heteropolyhedral framework structure, but the linkage is weaker in the c direction, accounting for the marked (001) cleavage.