X-Ray Identification of One-Layer Illite Varieties: Application to the Study of Illites Around Uranium Deposits of Canada

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Abstract
Structural and diffraction criteria for distinguishing between t-1 M, c-1M, m-1M, and 3T illite varieties are described. The t- 1M illite corresponds to a one-layer monoclinic structure with vacant trans- sites. The c-lM 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-lM illite corre- sponds to the structure in which cations are statistically distributed over available trans- and cis-sites. For t-lM, c-lM, and m-lM, the values of Ic cos #/al 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-lM instead. The relatively common occurrence ofc-lM 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.