Abstract: Illite from the Potsdam Sandstone (lower Ordovician) of Northwestern New York was studied by powder X-ray diffraction, scanning electron microscopy, and chemically analyzed and dated by the K/Ar method. The texture and ages of 360 and 392 Ma on two samples establish that the mineral is authigenic and relatively uncontaminated by Precambrian detritus. Random powder X-ray diffraction patterns show sharp and relatively intense 02l and 11l reflections, indicating an ordered structure. Comparisons with calculated patterns demonstrated that the mineral is not the common 1M or 2M1 polytype. Instead, the experimental pattern is very similar to that of a 3T polytype, but it agrees better with the calculated pattern of the octahedral cis-vacant, noncentrosymmetric (space group C2) structure found by Méring and Oberlin (1967) and Tsipursky and Drits (1984) in smectites, proposed for mica by Drits et al. (1984) and found by Zvyagin et al. (1985).