The first representative of the eudialyte group with a primitive unit cell has been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR spectroscopy. The mineral under study belongs to the system of sergevanite-raslakite-oneillite solid solutions; it is a low-symmetry variety of sergevanite, in which a high-valence zirconium cation compensates charge at the M2 site. The trigonal unit-cell parameters of the mineral are found to be a = 14.204(1) angstrom, c = 30.087(3) angstrom, and V = 5257.0(1) angstrom(3); sp. gr. P3; the number of independent sites is 165. The crystal structure is refined to the final reliability factor R = 4.9% in the anisotropic approximation of atomic displacements using 5620 reflections with F > 3 sigma(F). The idealized formula of the mineral (Z = 3) is Na15Ca3(Mn,Fe)(3)Zr-3[Na2Zr][Si26O72](O,OH)(3)Cl.H2O.
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