In order to explain why Tl-silicate minerals are extremely rare in natural occurrences, Tl-polyhedral comparisons between crystal structures of Tl-minerals and their isotypes were attempted qualitatively using the eccentricities and sphericities defined by the polyhedral-distortion analysis of the software IVTON. The structure data for Tl-minerals usually confirm the presence of Tl 6s~2 lone pair electrons causing distortion of the polyhedron: Tl~+ exhibits stereoactive lone pair behavior. Inasmuch as the silicate structures afford no accommodation for extremely distorted polyhedra, the cations with the lone pair electrons responding as incompatible elements during the crystallization of silicate magmas can be finally crystallized as the sulphides minerals where these cations require no extreme distortion. The Hard-Soft Acid and Base (HSAB) principle holding for more stable complexes facilitates an effective explanation for the reason that thalliums prefer sulfurs to oxygens. Furthermore the rule in the valence shell electron pair repulsion (VSEPR) model, that a nonbonding pair occupies more space on the "surface" of the central atom that a bonding pair, gives a significance to the orientation of lone pair electrons in Tl restricted by the software IVTON. The lone pair electrons responsible for extremely distorted Tl-polyhedra, namely stereoactive lone pair behavior of Tl~+, offer a substantial evidence for creating structural instability of the silicate minerals, leading to rare occurrences of these minerals containing the cation as a major component, resulting in about forty species of Tl-sulphide minerals. The presence of lone pair electrons imposing considerable constraints upon the kinds of Tl-minerals may provide a comprehensive explanation for mineralizing not as silicates but mainly as sulphides or sulphates in nature.
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