Compositional variation within some glauconites and illites and implications for their stability and origins

Textural, mineralogical and chemical data are presented for glauconites and illites from the Lower Cretaceous Glauconitic Sand Formation, Alberta, Canada. Single crystal analyses by scanning transmission electron microscopy (STEM) indicate that both glauconite (29 analyses) and illite (44 analyses) correspond very closely to ideal dioctahedral structures. Al dominates the octahedral sheet in illite. Very extensive A1 for Fe^III substitution was also found in some of the glauconites, much more than previously recorded. The dioctahedral mica structure is clearly very stable and flexible with respect to A1 for Fe^III substitution. Iron-poor glauconites were found associated with pyrite. We believe that glauconites tend to lose iron progressively during burial diagenesis, especially where pore water iron activities are low (high HS^? activity and low Eh). It seems probable that A1 for Si tetrahedral substitution also takes place during diagenesis with consequent loss of swelling properties. This is the same trend which is responsible for conversion of smectite to illite. Glauconite forms only where sediments reside for lengthy periods at the interface between oxidizing and reducing environments, where Fe³⁺ is transiently available in solution. Related marine and terrestrial nontronites also form only in similar ‘interface’ environments.