Thallium aluminosilicate, TlAlSiO4, synthesized hydrothermally is monoclinic with space group P21/n [a = 5.4095(3), b = 9.4232(7), c = 8.2629(6) Å, γ = 90.01(2)o, V = 421.20(6) Å3, Z = 4]. The crystal structure was refined to an R index of 3.8% based on 1852 observed unique reflections. The compound is a unique framework silicate with a topology similar to that of the tridymite structure. The TlO8 polyhedron resembles a truncated rectangular pyramid, and shares its edges with three adjacent AlO4 tetrahedra, three SiO4 tetrahedra, and six TlO8 polyhedra. Local understaturation at the Tl position suggested by bond-valence analysis implies that lone-pair electrons are present. The geometrical data indicate that the inert pair causes distortion of the Tl-polyhedron. Polyhedral distortion analysis using the software IVTON places the lone-pair parallel to , pointing to the largest base of Tl polyhedron. The rule in the valence shell electron pair repulsion model that a nonbonding pair occupies more space on the “surface” of the central atom than a bonding pair supports the orientation of inert-pair electrons in thallium provided by IVTON. The remarkable structure distortion caused by the inert-pair effect explains the rarity of Tl as a major element in silicate minerals because these cannot accommodate extremely distorted polyhedra. In contrast, about forty species of Tl-sulfide minerals exist because these structures are more flexible. Furthermore this effect probably explains why atoms such as Ge2+, Pb2+, Sn2+, Sb3+, and Bi3+, crystallize not as silicate phases but mainly as sulfide ones in nature.