New Data on the Isomorphism in Eudialyte-Group Minerals. X: Crystal Structure of the Intermediate Member of the Raslakite–Sergevanite Series

An intermediate member of the raslakite–sergevanite solid-solution series has been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR spectroscopy. The trigonal unit-cell parameters of the mineral are found to be a = 14.208(1) A, c = 30.384(1) A, and V = 5311.80(6) A3; sp. gr. R3. The crystal structure is refined to the final reliability factor R = 4.59% in the anisotropic approximation of atomic displacements using 3394 reflections with F > 3σ(F). The idealized formula of the mineral (Z = 3) is [Na12(H3O)2][Ca3(Fe1.5Mn1.5)]Zr3[Na2Zr][Si26O72]OH4. Splitting of the site of six-membered ring of octahedra, occupied mainly by Fe and Mn, has been established for the first time. The problems of the genesis of calcium-poor eudialyte-group minerals are discussed.