ILLITE FROM THE POTSDAM SANDSTONE OF NEW-YORK - A PROBABLE NONCENTROSYMMETRIC MICA STRUCTURE

Illite from the Potsdam Sandstone (lower Ordovician) of Northwestern N ew York was studied by powder X-ray diffraction, scanning electron mic roscopy, and chemically analyzed and dated by the K/Ar method. The tex ture and ages of 360 and 392 Ma on two samples establish that the mine ral is authigenic and relatively uncontaminated by Precambrian detritu s. Random powder X-ray diffraction patterns show sharp and relatively intense 02l and 11l reflections, indicating an ordered structure. Comp arisons with calculated patterns demonstrated that the mineral is not the common 1M or 2M1 polytype. Instead, the experimental pattern is ve ry similar to that of a 3T polytype, but it agrees better with the cal culated pattern of the octahedral cis-vacant, noncentrosymmetric (spac e group C2) structure found by Mering and Oberlin (1967) and Tsipursky and Drits (1984) in smectites, proposed for mica by Drits et al. (198 4) and found by Zvyagin et al. (1985).