The synthesis and structural characterization of AV-1 (Aveiro microporous solid no. 1), the first synthetic microporous yttrium silicate containing stoichiometric amounts of framework sodium (and yttrium) cations, are reported. AV-1, Na2K2Y2Si16O38. 10H(2)O, possesses the structure of the rare mineral montregianite. The synthesis and structural characterization of AV-2, an alkali calcium silicate hydrate (ideal composition HKCa2Si8O19.6H2O) analogue of the rare mineral rhodesite, are also reported. The structures of AV-1 and AV-2 are characterized by the same type of double silicate layers, but important differences exist with respect to the constitution of the octahedral layers and channel species. AV materials have been characterized by a range of techniques, viz., scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Si-29 and Na-23 single- and triple-quantum (3Q) magic-angle spinning (MAS) NMR spectroscopy, Fourier transform spectroscopy (FTIR), and thermogravimetric analysis (TGA).