As a consequence of the formation of these T4R units, the 20 MR pores in ZEO-5 are no longer fourlobe shaped but are open through a rhomboid window with no protruding obstacles and with a much wider clearance (14.3×13.5 Å along the diagonals, 10.7×10.7 Å across opposite sides; Fig. ). On the other hand, the 16 MR pores along directions normal to [001] are open through oblong 12.3×8.1 Å windows (Fig. ), more open than those in the precursors, specially ZEO-4A (Fig. ). Pore apertures of that size are unprecedented in stable, silica zeolites. Despite its very open framework ZEO-5 shows an outstanding stability that allows it to withstand calcination up to at least 1000 ºC and steaming under 10% H 2 O up to at least 760 o C, 3 hours (Fig. ), which is a consequence of the full connectivity and silica composition of its framework.