Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM)

Abstract

A detailed investigation of microstructures of authigenic illite has been carried out using highresolution TEM. Near-atomic images of 1M and 2M₁ polytypes of illite with incidence [100] and [110] were observed. We unambiguously determine 1M illite in both individual particles and microtwinned domains. Newly formed 2M₁ domains are surrounded by former packets of 1Md or 1M illite. Such intergrowth of different illite-polytypic domains on a scale of several layers suggests that they are metastable. The lateral-coherent 1M and 2M₁ illite domains are aligned along the same orientation and crystallographically continuous normal to [001]*. This is a new phenomenon observed for polytypic intergrowth and transition. The intralayer-stepwise change of stagger direction in a series of successive layers and consequently the interlayer-stacking angle change resulting in polytypic transition are hypothesized as an “intralayer-stepwise transition” model. It explains the process of illite polytypic transition in the solid state.