Metastable authigenic 1M illite from shale of diagenetic grade has been studied using a high-resolution transmission electron microscope (HRTEM) equipped with energy-dispersive spectrometer, X-ray diffraction, and scanning electron microscope. The illite occurs as deformed flakes deficient in interlayer K+ cations with 0.6 per half cell, and with abnormally high Al in both octahedral and tetrahedral sites. Complex structural adjustments reflecting the unusual chemical composition are observed in images of illite at near-atomic resolution. Different distances and directions of intralayer shift between the upper tetrahedral sheet and the lower tetrahedral sheet within 2:1 layers are found in this sample. Intralayer undershift structure coupled with interlayer displacement is found in a 1M illite crystal, and intralayer overshift structure coupled with no interlayer displacement is found in a 1M domain of a larger crystal. Two tetrahedral sheets across the interlayer region sometimes deviate from ideal positions causing interlayer displacement. Two pyrophyllite layers are found overlying a stack of ordered 1M illite layers, and are overlain by illite layers with anomalous interlayer offsets. This offset is considered to result from an increase in the lateral dimensions of the tetrahedral sheet due to anomalous high Al content. Our observation of intralayer and interlayer deficiencies indicate that authigenic illite that crystallized in the early stage of diagenesis at low temperatures tends to give rise to heterogeneous, disordered, and metastable structures.

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