Crystal-structure refinement of zirconium-rich eudialyte and its place among calcium-poor eudialyte-group minerals

Abstract

The repeated refinement of the crystal structure of zirconium-rich eudialyte based on the X-ray diffraction data set collected earlier revealed new structural features. The trigonal unit-cell parameters are a = 14.222(3) Å, c = 30.165(5) Å, V = 5283.9 ų. The refinement resulted in the reduction of the R factor from 0.045 (2347F > 4σ(F)) to 0.035 (3124F > 3σ(F)). It was found that the ordering of Ca and Fe in six-membered rings leads to the lowering of the symmetry to R3. An excess amount of zirconium (more than three atoms per symmetrically independent unit) is located in the M2 microregion in square and five-vertex polyhedral positions. However, this amount is insufficient to be dominant, and the deficiency of zirconium is compensated for by sodium atoms. Based on the new data, zirconium-rich eudialyte can be assigned to the oneillite subtype, being a zirconium-rich and aluminum variety of raslakite.