Four heteropolyhedral microporous silicates, A(3)RESi(6)O(15).2.25H(2)O, crystallizing in the Cmm2 space group and based on 4(1)5(1)6(1)8(2) tetrahedral sheets [A(3) = Na(2.74)K(0.26), RE = Ce, abbreviated as TR05; TR06: A(3) = Na(2.72)K(0.28), RE = La; TR07: A(3) = Na(3), RE = La; TR08: A(3) = Na(2.74)(H(3)O)(0.26), RE = La(0.68)Eu(0.32)] have been hydrothermally synthesized in Teflon-lined autoclaves at 503 K and structurally characterized using X-ray diffraction single-crystal data. Except for TR05, diffraction data have been collected on {001} twins by merohedry. The four structures are isotypic and based on strongly corrugated 4(1)5(1)6(1)8(2) silicate sheets interconnected along [010] by seven-coordinated RE polyhedra to form a microporous heteropolyhedral framework. The framework is crossed by three systems of ellipsoidal channels that host H(2)O molecules and alkaline ions. The channels run either parallel or perpendicular to the silicate sheets; the largest effective channel width is 4.7 x 2 A. In TR08 some (H(3)O)(+) replaces alkalis. Although the H atoms have not been localized, the configuration of the hydrogen bonding has been deduced from bond lengths and angles.