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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 transsites. 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 corresponds 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.

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The depositional setting of the 2·1 Ga fill of the Franceville Basin of Gabon is important for understanding the habitat (energy and availability of light and oxygen) and taphonomy of recently discovered early macro‐organisms buried in black shales in Unit FB. The available data bearing on the stratigraphy and sedimentology of Unit FB provide new insight into processes acting on the palaeo‐sea floor. The shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands. The latter (Poubara sandstones) were emplaced during a forced regression during the terminal infill of fault‐bounded sub‐basins following a stage characterized by a ferruginous to anoxic water column. The structureless sandstones were deposited from high‐density gravity currents along with a locally strong bottom oscillation of the water column. Tuft structures preserved in cyanobacterial mats, together with the position of the macro‐organisms at the top of the sandstone beds within associated black shales, point to a water depth of less than 80 m. The relative sea‐level fall that drove deposition of the Poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows.