The crystal structures of minerals and inorganic compounds with OPb₄ oxocentered tetrahedra are reviewed. It is shown that the OPb₄ tetrahedral units may link by sharing common Pb atoms to form structural units of various shape and dimensionality. These units determine basic topology of the structures and influence their stability and properties. The high strength of the OPb₄ te trahedral units involves interplay between high basicity of additional O^2– anions and stereochemical activity of the 6s ² lone electron pairs on Pb²⁺ cations. The structural chemistry of polycations based upon OPb₄ tetrahedra, in general, follows major trends previously observed for cation-centered tetrahedral units (silicates, phosphates, metal sulphides with MS₄ tetrahedra, etc.). One may conclude that the basic structural correlations depend upon size and charge parameters of the ions only, irrespective of their positive or negative sign.

The crystal structure of Pb₂₁[Si₇O₂₂]₂[Si₄O₁₃] has been solved on crystals grown by crystallization from melt. The compound is hexagonal, P6₃/m, a = 9.9244(5), c = 34.2357(16) Å, V = 795.28(6) ų, R₁ = 0.042 for 3361 unique observed reflections. The structure contains five symmetrically independent Si sites tetrahedrally coordinated by O atoms. The Si1O₄, Si3O₄, and Si4O₄ tetrahedra share corners to form branched heptameric [Si₇O₂₂]¹⁶⁻ units, whereas the Si2O₄ and Si5O₄ tetrahedra form the tetrameric [Si₄O₁₃]¹⁰⁻ anions. The structure contains six symmetrically independent Pb sites with the PbO_n coordination polyhedra distorted due to the stereochemical activity of the lone electron pairs. The structure can be described as a stacking of layers of the two types, A and B. The A-type layer contains [Si₇O₂₂]¹⁶⁻ units, Pb1, Pb2, Pb3, and Pb4 sites, whereas the B-type layer contains [Si₄O₁₃]¹⁰⁻ anions, together with Pb5, Pb6, and Pb6A sites. Stacking of the layers can be described as a sequence ...AA′BAA′B..., where A and A′ denote A layers with opposite orientations of the tripod-shaped silicate heptamers. The crystal structure of Pb₂₁[Si₇O₂₂]₂[Si₄O₁₃] has many similarities to that of hyttsjöite, which contains the same layers consisting of tripod-shaped [Si₇O₂₂]¹⁶⁻ anions. In both title compound and hyttsjöite, the anions are stacked together in such a way that ellipsoidal cavities with dimensions of ca. 10 × 6 × 6 ų are created. The cavities are occupied by the ClPb₆ octahedra in hyttsjöite and by “empty” Pb₆ octahedra in Pb₂₁[Si₇O₂₂]₂[Si₄O₁₃]. Analysis of structural and chemical complexity in the PbO-SiO₂ system indicates that the most chemically complex phases (in terms of complexity of relations between chemical components) appear to be the most complex from the structural point of view as well. The title phase is the most structurally and chemically complex phase in the system. Structural organization of crystalline phases in the PbO-SiO₂ system can be described as controlled by the Pb:Si ratio. For the phases with Pb:Si <2, their structures contain Pb²⁺ ions and silicate anions. For the phases with Pb:Si <2, the structures contain “additional” O atoms, i.e., atoms that are not bonded to Si. These atoms form OPb₄ tetrahedra, which are the next strongest structural subunits in the structure after silicate anions. The structures of the phases with Pb:Si <2 can therefore be described as based upon silicate anions and polynuclear cationic units consisting of edge- and corner-sharing OPb₄ tetrahedra.