Resolution of the Polytype Structure of Some Illitic Clay Minerals that Appear to be 1Md

G. S. Austin1, H. D. Glass2 and R. E. Hughes2
1 New Mexico Bureau of Mines and Mineral Resources, Campus Station, Socorro, New Mexico 87801
2 Illinois State Geological Survey, 615 East Peabody Drive Champaign, Illinois 61820

Abstract: Illite/smectite (I/S) and illite samples that appear to be 1Md polytypes on the basis of the lack of diagnostic reflections can be resolved as partly either 1M or 2M1 or both by either vapor solvation in ethylene glycol for 40 to 80 hr at ambient temperatures or for 4 hr at 80°C, or by heating to 375°C for 2 hr. If samples are so treated, a distinct narrowing of the I/S 003/005 peak occurs and diagnostic hkl reflections are revealed. These treatments, however, do not seem to affect diagnostic polytypic peak intensities, but rather the adjacent background. We propose that the 1Md term be restricted to its original definition—the disordered polytype associated with only the 1M ordered polytype. For illites in which disorder exists with a mixture of polytypes, the generic term Ad (disordered group A micas) should be used. Correct polytype determination of illitic material is significant because it reveals thermal history of the sediments and allows greater understanding of the depositional and diagenetic history of sedimentary basins.

Key Words: Ethylene glycol solvation • Illite • Illite/smectite • Polytype • Order-disorder • X-ray powder diffraction

Clays and Clay Minerals; April 1989 v. 37; no. 2; p. 128-134; DOI: 10.1346/CCMN.1989.0370203
© 1989, The Clay Minerals Society
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