The uses of zeolites are limited by the size of their pores, so zeolites with larger pores are in demand . Natural and synthetic zeolites possess a fully connected three-dimensional network of corner-sharing SiO4 tetrahedra (i.e. they are tectosilicates or framework silicates , with Si occasionally substituted by other atoms). However, in some cases zeolites are obtained in the form of two-dimensional precursors (phyllosilicates or layered silicates) (6) that only become fully connected tectosilicate zeolites by condensation of their layers through a calcination procedure that is "topotactic" because it does not alter the layer topology . The condensing layers can be obtained by direct synthesis or by disassembly of certain zeolites as in the so-called ADOR (assembly-disassembly-organization-reassembly) process . However, in around eight decades of extensive and systematic zeolite synthesis studies , there has been no example or prediction of a three-dimensional (3D) zeolite obtained by condensation from a one-dimensional (1D) precursor, either directly synthesized or obtained by disassembly of another zeolite. We report here the first case of such a 1D-to-3D topotactic condensation from ZEO-2, a directly synthesized complex "zeolitic" chain silicate, into ZEO-3, a fully connected three-dimensional extra-large pore zeolite (ZEO-n refers to materials discovered and patented by the Anhui ZEO New Material Technology Co., China). This condensation does not alter the topology of the chain silicate, so it is topotactic. The resulting zeolite ZEO-3 is outstanding for a number of reasons, including its very low density, its multidimensional system of interconnected extra-large pores, and the presence in its structure of double four-member ring units (D4R), i.e. small cubes of silica. For pure silica zeolites, this kind of unit is strained and up to now was believed to need a fluoride anion close to its center to be accessible for crystallization since it has never been seen before in a silica zeolite synthesized without the use of fluoride anions.