Crystal structures of four natural Na-rich cordierites (space group Cccm) refined at room temperature are described. Electron-microprobe, Li and Be analyses, and structure refinements indicate the substitutions Na+ + Be2+ → Al3+ and Na+ + Li+ → Mg2+ or Fe2+. All crystals exhibit a high degree of (Si,Al) ordering typical of low-cordierites. The effect of Na on the structure is determined by comparison with previously reported structures of Na-poor crystals. Partial incorporation of Na in the center of the six-membered ring of oxygen atoms (O23-O26-O21-O23-O26-O21) causes the rings to predominantly compress along the b-axis, whereas the larger six-membered rings of cations (T23-T26-T21-T23-T26-T21) slightly expand along the same direction. These opposing motions lead to an increase in the b-axis and are responsible for a nearly hexagonal lattice geometry in Na-rich cordierites. Linear regression analyses reveal that tetrahedral angles (O-T-O) and individual T-O bonds are affected by Na content. The Al tetrahedron T1 1, interconnecting six-membered rings of tetrahedra, is the preferred site for Be substitution in cordierite. The smaller ionic radius of Be significantly decreases the mean T1 1-0 bond length. Li partially occupies octahedral positions and substitutes for Mg or Fe. All Na-rich cordierites studied are also H2O rich because each Na attracts two water molecules into the adjacent structural cavities to complete its coordination polyhedron of eight oxygen atoms.