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The crystal structure of synthetic TIAISi₃O₈

Institute of Geoscience, University of Tsukuba, Tennodai 1–1-1, Tsukuba, Ibaraki, 305–8571, Japan

^* e-mail: kyono{at}esca.geo.tsukuba.ac.jp

The crystal structure of synthetic Tl feldspar, TIAISi₃O₈, was determined based on single-crystal X-ray diffraction data; monoclinic, a = 8.882(3), b = 13.048(2), c = 7.202(2) Å, β = 116.88(1)°, V = 744.5(4)ų, Z = 4, space group C2/m (R = 7.33% for 462 observed reflections). The underlying framework is similar to Rb feldspar, RbAlSi₃O₈, and can be regarded as isotypic with sanidine. The structure accommodates Tl⁺ cations occupying the M site coordinated by nine O atoms. The more expanded Tl polyhedra as compared to the Rb polyhedra, in spite of the small ion radius of Tl⁺ relative to Rb⁺, result from the behavior of the stereoactive lone-pair electrons of Tl⁺ called "inert-pair effect". The difference in centroid-central atom distance in the TlO₉ polyhedron suggests that the lone-pair electrons are orienting parallel to the [001] direction in the feldspar structure. Therefore, Tl found as a trace element in analyses of natural K-feldspar can be considered as an actual constituent of this mineral. The syntheses of the present Tl-feldspar, TlAlSi₃O₈, together with Tl-leucite, TlAlSi₂O₆, and the chemical analogue of kalsilite, TlAlSiO₄, shed new light on the crystal chemistry of potassium aluminosilicate minerals.

Key-words: thallium, feldspar, crystal structure, lone-pair electrons, stereochemical activity, inert-pair effect.