DISSYMMETRIZATION IN EUDIALYTE-GROUP MINERALS. V. CRYSTAL STRUCTURE OF POTENTIALLY NEW MINERAL FROM MOROCCAN PEGMATITE – FIRST HIGH CALCIUM MEMBER WITH Р3-SYMMETRY

A study of the crystal structure of high-calcium eudialyte from the Tamazert massif (Morocco) revealed features of its structure that reduce the symmetry of the mineral. The parameters of the trigonal cell are: a = 14.1863(1), c = 30.0703(2) Å, V = 5240.95(4) ų, sp. gr. P3. The structure, the unit cell of which contains 161 independent positions of atoms, is refined to the final divergence factor R = 4.4% in the isotropic approximation of atomic displacements using 8202 reflexes with F > 3σ(F). The ordering of Ca and Mn atoms in the octahedra of the six-membered rings, Fe and Mn in the M2 positions between the translation-bound rings, as well as Na, H₃O, La and K in the extra-framework N positions has been established. The studied mineral is chemically similar to golyshevite and feklichevite, from which it differs in a lower calcium content and a higher manganese content. The main features of the mineral are discussed in comparison with the previously studied high-calcium minerals of the eudialyte group within the framework of the R3 sp. gr. It is shown that a decrease in the symmetry of the structural model of high-calcium representatives of the eudialyte group may be due to the ordering of cations inhabiting the N-positions of the extra-framework part of the structure.