Single crystals of melilite-related tetrahedral sheet structures in the system CaO-ZnO-GeO2-SiO2 were grown by hydrothermal methods, sinter experiments, flux, and topotacti-cal techniques. Their crystal structures were determined and refined using X-ray singlecrystal data.

The high-temperature polymorph, high-Ca2ZnGe2O7, possesses an incommensurate structure at room temperature. The average structure with space group P421m, a = 7.950(1), c = 5.186(1) Å is of melilite type with layers formed by ZnO4 and GeO4 tetrahedra. Anisotropic displacement parameters indicate that the modulation may be interpreted in terms of librations of tetrahedral units about the c axis.

Low-Ca2ZnGe2O7, grown from a Na2W04 flux or by an epitaxial method, also possesses an incommensurate fine structure. The refined average structure, space group P21, a = 8.020(2), b = 7.995(2), c = 15.506(3) Å, β = 89.47(2)° displays a stacking variant of melilite-like layers with an ABA'ABA' stacking sequence. The stacking fragment A'A is essentially a melilite unit. Layer B is rotated 90° about c relative to layer A. Ca atoms between layers A and B have a disordered arrangement.

A new tetrahedral sheet structure with four-, five-, and six-membered rings of tetrahedra was synthesized at the lowest temperatures. The structure has space group P21/n, a = 9.112(1), b = 7.900(2), c = 9.380(1) Å, β = 114.03(1)° and is well ordered without any indication of modulation.

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