Transmission and analytical electron microscopic study of mixed-layer illite/smectite formed as an apparent replacement product of diagenetic illite

Ordered illite/smectite (I/S) and illite in a pelitic rock from a prograde metamorphic sequence in North Wales were observed by transmission electron microscopy. The dominant phyllosilicate noted was diagenetic-metamorphic illite, occurring as subparallel packets of layers, each about a few hundred Aangstroms thick. It exhibited two-layer polytypism (presumably 2M (sub 1) ) and numerous strain features and had a composition of (K (sub 1.21) Na (sub 0.12) )(Al (sub 3.36) Fe (sub 0.31) Mg (sub 0.33) )(Si (sub 6.28) Al (sub 1.72) )O (sub 20) (OH) (sub 4) . I/S occurred as thick packets of wavy layers having 10-Aa subperiodicity and sharp differences in contrast in successive lattice fringes. All stages in a replacement series were noted, from one or two layers of smectite within illite, through thin packets of I/S, to thick packets that contained inherited deformation textures of diagenetic-metamorphic illite. Deformed illite was replaced by I/S more commonly than was undeformed illite. The I/S replacing undeformed original illite had significantly greater order, primarily of R1 type (ISISIS...), than that replacing deformed illite. R> 1 I/S occurred as small crystallites and contained relatively less smectite than the ordered I/S, Single smectite layers were spaced within several illite layers, forming curved packets of layers. IISIIS... (R2) and IIISIIIS... (R3) ordering were present locally, as was discrete smectite. Analytical electron microscopic analyses indicated that the I/S, (K (sub 0.46) Na (sub 0.43) )(Al (sub 3.75) Fe (sub 0.06) Mg (sub 0.19) )(Si (sub 6.26) Al (sub 1.74) )O (sub 20) (OH) (sub 4) , was rectorite-like in composition and had smaller (Mg + Fe) contents and greater Al/Si ratios than the coexisting illite, which was also anomalous in terms of general crystal-chemical relationships between coexisting illite and I/S in burial diagenesis environments. The I/S appears to have formed by replacement of diagenetic-metamorphic illite, presumably at very low temperatures under hydrous conditions via dissolution and crystallization.