Structural and diffraction criteria for distinguishing between t-1M, c-1M, m-1M, and 3T illite varieties are described. The t-1M illite corresponds to a one-layer monoclinic structure with vacant transsites. The c-1M illite has vacant cis-octahedra forming one of two symmetrically independent point systems; the other cis-octahedra as well as the trans-octahedra are occupied; and the m-1M illite corresponds to the structure in which cations are statistically distributed over available trans- and cis-sites. For t-1M, c-1M, and m-1M, the values of |c cos β/a| are equal to 0.39–0.41, 0.29–0.31, and 0.333, respectively. Application of these criteria demonstrates that illite samples described in the literature as the 3T polytype usually are c-1M instead. The relatively common occurrence of c-1M illite in association with t-1M and 2M1 polytypes has been recognized in illite from hydrothermal alterations around uranium deposits located in the Athabasca basement (Saskatchewan, Canada). The c-1M illite from these deposits was previously described as 3T one.