Compositional and structural features of the octahedral sheet in mixed-layer illite/smectite from bentonites

We studied mixed-layer illite/smectite (I/S) from 28 bentonite samples using X-ray diffraction, electron-microprobe analysis, thermogravimetry and infrared spectroscopy in order to identify the structural and chemical evolution of the octahedral sheet during the reaction of smectite to illite. Studied samples range from smectitic to highly illitic I/S. Bentonite samples were from different locations in the USA and Europe, and have geologic ages ranging from Pliocene to Ordovician. Al and Fe exchange for each other in the octahedral sheet. With increasing illite, the I/S octahedral sheet becomes more Al-rich and Fe-poor, and Mg approaches a value of 0.5 per O₂₀(OH)₄. We attribute these trends to be the balance of electrostatic and structural controls. Smectitic I/S tends to be cis-vacant and becomes increasingly trans-vacant with increasing illitization. Cis-vacant I/S dehydroxylates at a higher temperature (650°C) compared to trans-vacant I/S (550°C). Octahedral Fe shows a tendency towards segregation; however, the exact extent of this segregation could not be determined. Fe segregation increases with increasing development of trans-vacancies.