Dissymmetrization in Eudialyte-Group Minerals: V. Crystal Structure of Potentially New Mineral from Moroccan Pegmatite—First High-Calcium Member with P3 Symmetry

A study of the crystal structure of the high-calcium eudialyte from the Tamazeght massif (Morocco) revealed some structural features reducing the symmetry of this mineral. The trigonal unit-cell parameters are a = 14.1863(1) Å, c = 30.0703(2) Å, V = 5240.95(4) Å3, sp. gr. P3. The structure, whose unit cell contains 161 independent atomic sites, was refined to the final residual factor R = 4.4% in the isotropic approximation of atomic displacements using 8202 reflections with F > 3σ(F). Ordering of Ca and Mn atoms in octahedra of six-membered rings; Fe and Mn atoms in М2 sites between translation-linked rings; as well as Na, Н3O, La, and K in extraframework N sites was established. The chemical composition of the mineral studied is close to those of golyshevite and feklichevite, but differs from them by a lower calcium content and a higher manganese content. The main features of the mineral in comparison with the high-calcium eudialyte-group minerals investigated previously within the sp. gr. R3 are discussed. It is shown that the symmetry lowering in the structural model of high-calcium eudialyte-group representatives may be caused by the ordering of the cations occupying N sites of the extraframework part of the structure.