Condensation of ZEO-4A and ZEO-4B into ZEO-5 results, paradoxically for a "condensation", in a significant expansion of the unit cell volume (12% and 7.5% expansion, respectively, Tab. S23), which is due to the longer size of the T4R unit compared to their precursors, lau or D6R, along the diagonals of the unit cell (Fig. ), while the c parameter changes less than 0.5% as a result of the invariance of the basic ZEO-2 chain running along [001] in the zeolite. The new ZEO-5 zeolite holds the record for a low framework density silica polymorph (FD) at 11.08 Si atoms per nm³, well below the prior recordholder, ZEO-3 (from which ZEO-5 is a σ-expansion, see Fig. ), with FD = 12.76 Si nm -³. This translates into a calculated density, 1.10 g cm ³, which is almost that of water. Furthermore, among all ⁻ the Zeolite Framework Types, irrespective of composition, connectivity or stability, included in the Database of Zeolite Structures, only RWY has a lower framework density (7.6 T nm -³). The RWY topology, containing 12-R pores and very large cages, is not a zeolite oxide but is only realized in UCR-20, a sulfide of gallium and germanium with an extremely poor thermal stability.