Investigation of Illite/Smectite Interstratification By X-Ray Diffraction and Transmission Electron Microscopy; Sedimented Aggregate Structure and Smectite to Illite Conversion in Taranaki Basin Sediments

Abstract

A suite of sediments from the Taranaki Basin region of New Zealand and some pure illite/ smectites have been studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). These examinations have identified the structure of illite/smectite sedimented aggregates, the origin of small fundamental particles, the mechanism of ordering development in illite/smectite, and the existence of an illitisation reaction in the Taranaki Basin. A pure rectorite clay was examined by HRTEM both in a crystalline and a dispersed state. The crystalline rectorite comprises large MacEwan crystals containing interstratification of illite and smectite layers. The dispersed rectorite comprises aggregates of thin fundamental particles, each only a single unit cell wide. These fundamental particles are formed as an artifact of specimen treatment. Despite an absence of large interstratified crystals, the dispersed rectorite continues to show interstratification of illite and smectite interlayers when examined by XRD. All of the Taranaki Basin samples were found to contain illite/smectite minerals. XRD analyses indicate that interlayering of the component layers in these illite/smectites range from random (R=0) in shallow buried sediments to non-nearest neighbour ordered (R>1) in deeply buried sediments. HRTEM experiments show the illite/smectite aggregates to be composed mostly of fundamental particles and a few MacEwan crystals. Most interstratified sequences of illite and smectite layers comprise fundamental particles stacked together in turbostratic parallel and subparallel alignments. Atomic models of the aggregates were constructed from quantitative fundamental particle data and were used to calculate theoretical XRD profiles. The profiles reproduce peak migrations and intensity fluctuations observed in experimentally recorded XRD patterns, confirming the appropriateness of a fundamental particle model for describing interstratification in illite/smectite aggregates. Rock fragments were sectioned and examined by HRTEM to establish possible origins for the fundamental particles. The undisturbed illite/smectite components in these rocks comprise anhedral crystals, large MacEwan crystals, and a minor proportion of independent thin illite. Most fundamental particles are likely to be secondary crystallites, formed by the disaggregation of the anhedral crystals and the MacEwan crystals. The absence of large amounts of independent thin illite particles in the sediments, indicates primary crystals may not be a major source of fundamental particles. Fundamental particle distribution data were used to calculate compositional parameters and junction probability statistics describing illite/smectite interlayering. These statistics indicate that there is a late development of R=l order, caused by the presence of thick fundamental particles in smectitic clays. Clays containing <40% illite layers have segregated illite and smectite interlayering. R=l ordering develops at>55% illite layers. Partial R=2 ordering develops at >60% layers. Imperfect R=l and R=2 ordering is caused by a persistence of thin fundamental particles in illitic clays. Fundamental particle thickening during burial in the Taranaki Basin causes illitisation of the illite/smectite during sediment diagenesis. The illitisation reaction proceeds in three stages; an initial slow illitisation in shallow sediments, a rapid illitisation between 3000-4000m burial, and a third slow illitisation below 4000m. Comparison of the illitisation kinetics with organic maturation data indicates that the illitisation reaction may be a poor indicator of diagenetic grade in Taranaki Basin sediments. Powder XRD analyses indicate that the Taranaki Basin sediments contain mica and plagioclase feldspar minerals, both of which decrease in concentration during burial; indicating that they are a possible source of ionic K+ and A13+ that is consumed by the illitisation reaction. The pattern of water release during burial, from smectite interlayers, was identified from thermodynamic calculations and illitisation kinetics. A substantial volume of interlayer water is released between 3000m and 4000m depth. This is coincident with fluid overpressuring in the Taranaki Basin sediments. It is probable that the fluid released from the illite/smectite clays has directly contributed to this overpressuring.