Abstract: The reaction of smectite to illite in shale from the COST 1 well in the south Texas Gulf Coast and from altered ash-fall tufts from the Morrison Formation in New Mexico was investigated using X-ray powder diffraction in conjunction with transmission electron microscopy. In the COST 1 well, the bulk of the detrital clay was originally a K+ mixed-layer illite/smectite (I/S). As the I/S adsorbed K+ released by the dissolution of K-feldspar during burial, the proportion of expandable layers decreased with depth from ∼65% near the top of the well to ∼25% at 4500 m depth. In contrast, the proportion of low-charged structural planes [<0.8 eq per (Al,Si)4O10 unit] in the I/S decreased gradually from ∼40% near the top of the well to ∼15% near the bottom. Authigenic smectite with 100% expandable layers from the Morrison Formation tufts is an alteration product of vitric ash. Where these tufts have been buried to ∼1400 m the smectite has reacted to form I/S with ∼15% expandable layers.
Direct lattice images of I/S crystallites from both locations reveal a correspondence between edge dislocations and the interface between illite layers and smectite layers. Al, Si, Fe, Ca, Mg, and Na were apparently mobile along these dislocations as the reaction of smectite to illite proceeded. Al was probably retained within the crystallite when illite layers replaced the smectite layers; however, some of the remaining cations were expelled. Lateral replacement of smectite layers by illite appears to have been the principal growth mechanism.