The crystal structure of girvasite, NaCa2Mg3(PO4)3(CO3)(H2O)6, has been refined using X-ray diffraction data collected at 173 K, which allowed us to revise its crystal chemical formula. The mineral is monoclinic, P21/c, a = 6.4784(2), b = 12.2313(3), c = 21.3494(6) Å, β = 89.624(2)º, V = 1691.67(8) Å3 (at 173 K), Z = 4, R1 = 0.037 for 6471 unique observed reflections. The crystal structure of girvasite contains three Mg, two Ca, and one Na sites. The Mg atoms are octahedrally coordinated by O atoms and H2O molecules. The Ca sites are coordinated by eight anions each, whereas the Na site has a coordination number equal to seven. The crystal structure is based upon heteropolyhedral sheets formed by polymerization of Mg octahedra, PO4 tetrahedra, and CO3 groups. The sheets consist of fundamental building units (FBUs) formed by two Mg octahedra that share edges to form dimers decorated by three PO4 tetrahedra and linked to a unit consisting of a MgO6 octahedron sharing an edge with a carbonate triangle. The FBUs