AbstractThermal behavior of catapleiite, ideally Na2ZrSi3O9 ⋅ 2H2O, has been investigated with the X-ray diffraction, electroconductivity measurements using impedance spectroscopy, dynamic scanning calorimetry, thermal gravimetry, and infrared spectroscopy. The high-temperature transformation product of catapleiite obtained after annealing of catapleiite at 1000°C has been studied. This is a new microporous zirconosilicate, ideally Na6Zr3[Si9O27], representing a new structural type. It is hexagonal, P63/mcm, a = 11.5901(9), c = 9.9546(9) Å, V = 1158.05(16) Å3. Its crystal structure model has been obtained using single-crystal X-ray diffraction data and refined by the Rietveld method on the basis of the powder X-ray diffraction data (R = 3.87%). The structure is based on the heteropolyhedral framework, which substantially differs from that of catapleiite and is built by nine-member rings Si9O27 formed by SiO4 tetrahedra linked with isolated ZrO6 octahedra. Extraframework Na cations are located in broad channels of the framework and between the tetrahedral Si9O27 rings.