Rk. Rastsvetaeva et al., Crystal structure and crystal-chemical features of a new Ti-rich member ofthe eudialyte family, Z KRISTALL, 214(5), 1999, pp. 271-278
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.