| Summary: | Combined X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were used to identify and characterize the clay mineral assemblages in 0.05-0.1 $ mu$m and ${<0.05} mu$m size fractions separated from shales of three depths (4800, 8700-8750, and 12500-12550 ft) from the Reindeer D-27 well in the Beaufort-Mackenzie area, Arctic Canada. Ethylene-glycol solvated samples indicated a two-component mixed-layer I/S system, whereas glycerol solvation suggested an additional high-charge expandable component. Exchange with octylammonium (n$ rm sb c$ = 8) and octadecylammonium (n$ rm sb c$ = 18) ions revealed the presence of five distinct types of layer structures: (1) low-charge smectite-group, (2) high-charge smectite-group or vermiculite, (3) double-layers or rectorite-like, (4) expandable illite, and (5) non-expandable illite. The presence and abundance of each type of layer structure changed with burial depth. A comparison of untreated and sodium hexametaphosphate-treated ${<0.05} mu$m size fractions revealed modifications to the structure of mixed-layer I/S in both ethylene-glycol solvated and alkylammonium-ion exchanged specimens. Changes in peak position, intensity and breadths are apparent in XRD, and these modifications can also be observed in HRTEM. Lattice-fringe images revealed that pretreatment resulted in: (1) increased abundance of 2:1 layer silicate packets with expanded interlayers, (2) increased R1-ordered sequences, and (3) thinner packets of non-expanded illite interlayers. The use of sodium hexametaphosphate as a peptizer alters the interlayers and modifies the stacking arrangement of the layer structures. If the primary structure is altered, the disruption may be irreversible. It is evident therefore, that reliable ratio determination between expandable and non-expandable components in mixed-layer I/S is dependent on characterizing all the components and resolving the effects of chemical pretreatment.
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