Crystal structure and crystal-chemical features of a new Ti-rich member of the eudialyte family

The trigonal structure of a new member of the eudialyte group was established from X-ray single-crystal diffraction, lambda(MoKalpha) = 0.71073 Angstrom; space group R3m; a = 14.153(9) Angstrom, c = 60.72(5) Angstrom, V = 10546.1 Angstrom(3); M = 3050; D-x = 2.88g/cm(3); Z = 3; mu = 28 cm(-1); R(hkl) = 0.068 for 3402 reflections. Due to the ordered replacement of Zr with Ti, the new mineral is characterized by a doubling of c and an alternation along [001] of two microdomains, containing (Zr, Ti) or (Ti, Zr)-octahedra, respectively. The key positions on the triad axis are accommodated by Si, Mn and Al atoms. Mn is four-fold coordinated and dominates in one nine-membered ring of the Ti microdomain while Al is six-fold coordinated in the ring of the Zr microdomain. The second key sites are related to Fe atoms which are situated in a slightly aplanar oxygen squares similarly to other relevant examples. These two positions are completed with some neighbour subsites. The axial Al octahedron is connected with three Ti octahedra around the triad axis forming a polyhedral cluster. The splitting and isomorphic substitution of the Na dominant positions are similar to other eudialyte minerals. It was also reported for kentbrooksite. The Na(8b) and Na(9b) atoms are the most disordered around the 18c site with 25% occupancy. The unusual doubling of the structure in the new mineral may be interpreted as a commensurate modulation with wave displacements along the c axis. Other original features of this new representative in the eudialyte family are also discussed. [References: 19]