Publication

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

Gervais Chapuis
1999
Article
Résumé

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]

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Concepts associés (32)
Structure cristalline
La structure cristalline (ou structure d'un cristal) donne l'arrangement des atomes dans un cristal. Ces atomes se répètent périodiquement dans l'espace sous l'action des opérations de symétrie du groupe d'espace et forment ainsi la structure cristalline. Cette structure est un concept fondamental pour de nombreux domaines de la science et de la technologie. Elle est complètement décrite par les paramètres de maille du cristal, son réseau de Bravais, son groupe d'espace et la position des atomes dans l'unité asymétrique la maille.
Silicate mineral
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually considered a silicate mineral. Silica is found in nature as the mineral quartz, and its polymorphs. On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that have been forming and re-working the crust for billions of years.
Cristallographie aux rayons X
La cristallographie aux rayons X, radiocristallographie ou diffractométrie de rayons X (DRX, on utilise aussi souvent l'abréviation anglaise XRD pour X-ray diffraction) est une technique d'analyse fondée sur la diffraction des rayons X par la matière, particulièrement quand celle-ci est cristalline. La diffraction des rayons X est une diffusion élastique, c'est-à-dire sans perte d'énergie des photons (longueurs d'onde inchangées), qui donne lieu à des interférences d'autant plus marquées que la matière est ordonnée.
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