The small-pore framework stannosilicate Na2SnSi4O11.2H2O (Sn-AM-3), exhibiting the structure-type of the rare mineral penkvilksite-2O, has been synthesized via a seeded hydrothermal process. The estimated cell dimensions are: a = 16.52, b = 8.80, c = 7.48 Å. The Sn-AM-3 three-dimensional framework is made up of [Si4O11] silicate sheets interconnected by SnO6 octahedra. The [Si4O11] silicate sheets may be described as a combination of the spiral chains formed by the corner-sharing SiO4 tetrahedra. Sn-AM-3 is the first example of a synthetic microporous stannosilicate whose structure is built up from silicate sheets. All materials were studied by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, 29Si, 119Sn, single-and triple-quantum 23Na magic-angle spinning NMR spectroscopy and thermogravimetry. Solid-state NMR is a particularly powerful technique to establish the presence of the 1M and 2O polytypes and accurately determine their relative abundances in mixed samples.