TEM OBSERVATIONS OF COHERENT STACKING RELATIONS IN SMECTITE, I S AND ILLITE OF SHALES - EVIDENCE FOR MACEWAN CRYSTALLITES AND DOMINANCE OF 2M(1) POLYTYPISM/

TEM characterization of stacking relations in I/S of expanded shale sa mples from the Gulf Coast and Michigan Basin was carried out to addres s the issues of the degree of coherency and the nature of layer stacki ng sequences in smectite, I/S and illite. The two-dimensional lattice fringe images obtained from this study show that cross fringes are com monly observed to be continuous over at least 3-4 layers for smectite, 6-7 layers for ordered I/S and 9-10 layers for illite-rich I/S. This demonstrates that such sequences are coherent, or at least semi-cohere nt (in smectite) units (MacEwan crystallites). The observations demons trate that so-called fundamental particles are fragments of MacEwan cr ystallites formed primarily as a result of disaggregation along weakly -bonded smectite interlayers. However, both Okl and hOl reflections ma y coexist in selected area electron diffraction (SAED) patterns. The f requency of occurrence of the coexistence in SAED patterns decreases i n the order Smectite, I/S and illite for Gulf Coast samples. This is c onsistent with the presence of turbostratically-related interfaces in packets of all of these materials. Therefore, any given layer sequence in smectite, ordered I/S and illite may have both turbostratic and co herent interfaces. The proportion of coherently-related layers increas es with increasing proportion of illite-like layers. The concept of fu ndamental or elementary particles is not related to layer sequences in non-disaggregated, original rocks. Indeed, it implies relations that are not valid. The lattice fringe images, SAED and optical diffraction patterns demonstrate that where layers are coherently-related, 2M(1) is the dominant polytypic sequence in all samples. However, this perio dic 2M(1) stacking is so frequently interrupted by stacking faults in smectite that it gives rise to apparent 1M(d) polytypism. The degree t o which the periodic 2M(1) sequences are interrupted by stacking fault s decreases with increasing proportion of illite-like layers. The SAED patterns of I/S and illite unmodified since its formation are diffuse parallel to c and have poorly-defined, non-periodic reflections for indices k not equal 3N as a measure of local ordering superimposed on poorly-ordered coherent sequences with a turbostratic component. X-ray diffraction (XRD) patterns, as integrated over domains with a range o f heterogeneous stacking relations, do not represent simple mixtures o f discrete 1M and 2M(1) polytypes. The observations of this study impl y that dissolution-crystallization is a dominant mechanism for the sme ctite-to-illite transition. The semi-coherent stacking of smectite-lik e layers in smectite-rich samples implies that either a dissolution-cr ystallization process took place subsequent to deposition of detrital smectite or that Gulf Coast smectite is an in-situ alteration product of volcanic ash.