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

The trigonal structure of a new member of the eudialyte group was establish ed from X-ray single-crystal diffraction, lambda(MoKalpha) = 0.71073 Angstr om; space group R3m; a = 14.153(9) Angstrom, c = 60.72(5) Angstrom, V = 105 46.1 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 T i, the new mineral is characterized by a doubling of c and an alternation a long [001] of two microdomains, containing (Zr, Ti) or (Ti, Zr)-octahedra, respectively. The key positions on the triad axis are accommodated by Si, M n and Al atoms. Mn is four-fold coordinated and dominates in one nine-membe red 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 releva nt examples. These two positions are completed with some neighbour subsites . The axial Al octahedron is connected with three Ti octahedra around the t riad axis forming a polyhedral cluster. The splitting and isomorphic substi tution 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 t he most disordered around the 18c site with 25% occupancy. The unusual doub ling of the structure in the new mineral may be interpreted as a commensura te modulation with wave displacements along the c axis. Other original feat ures of this new representative in the eudialyte family are also discussed.