The crystal structure of synthetic TlAlSi (sub 3) O (sub 8) ; influence of the inert-pair effect of thallium on the feldspar structure

The crystal structure of TlAlSi (sub 3) O (sub 8) (a 8.882, b 13.048, c 7.202 Aa, beta 116.88 degrees , V 744.5 Aa (super 3) , Z = 4, space group C2/m) was determined to R 7.33%. The underlying framework is basically similar to that of RbAlSi (sub 3) O (sub 8) and can be regarded as isotypic with sanidine. The structure accommodates Tl (super +) occupying the M site coordinated by nine O atoms. The more expanded Tl polyhedra, compared to the Rb polyhedra (despite the small ionic radius of Tl (super +) relative to Rb (super +) ), result from the behaviour of the stereoactive lone-pair electrons of Tl (super +) termed 'inert-pair effect'. The difference in centroid-central atom distance in the TlO (sub 9) polyhedron suggests that the lone-pair electrons are orienting [001] in the feldspar structure. Therefore, Tl found as a trace element in natural K-feldspars can be considered as a true constituent of the mineral.