Thallium leucite, TlAlSi 2 O 6 , has been synthesized at 450 degrees C for 7 days, under ambient conditions, by the transformation of dehydrated analcime NaAlSi 2 O 6 in the presence of excess T1C1. This substitution of T1 for Na leads to confirmation of a thallium-leucite pseudomorph after analcime. Their optical properties, X-ray powder diffraction patterns, electron microprobe analysis, infrared spectra, and X-ray photoelectron spectroscopy have characterized the synthetic T1-leucites. The IR spectra show that the mid-IR modes T-O stretching and T-O-T bending vibrations for TlAlSi 2 O 6 are more resemblant of those for analcime than for leucite, KAlSi 2 O 6 . This resemblance implies that T1 cation enters the W-site rather than the S-site in the analcime structure: Na (S)+H 2 O (W)<-->+K (leucite)<-->+Tl (Tl-leucite), where  represents an S-site vacancy. The mechanism of this substitution is supported by the crystal chemical constraints: inasmuch as the S-site is smaller than the W-site, Tl (super +) cations being larger than Na (super +) plainly prefer the latter site to the former. One inference from the binding energy for Tl (super +) by XPS is that Tl (super +) occupies the extra-framework site in synthetic leucite pseudomorph, rather than the smaller tetrahedral site. The difference in A1/Si disordering between analcime and leucite and the nonstoichiometry due to the solid solution of the Si 3 O 6 component into the leucite structure may provide a fundamental insight into understanding why TlAlSi 2 O 6 deviates from the trend defined by K-, Rb- and CsAlSi 2 O 6 leucite series on the a-c parameter diagram, inasmuch as these three cations in the leucite structure occupy the W-sites. Finally, synthesis of TlAlSi 2 O 6 leucite has an implication for the existence of other polymorphs due to different degrees of Al/Si disordering, except for high- and low-temperature leucites already known: natural leucites crystallized directly through igneous processes are different from those formed by substitution of K for Na in analcimes.