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A B S T R A C T. A range of authigenic sedimentary chloritesfrom sandstones has been studied by analytical transmission electron microscopy. Selected area (single crystal) electron diffraction patterns are of the Ib (fl = 90 ~ polytype confirming the earlier observations of Hayes (1970).TEM analyses show all samples to be relatively rich in both A1 and Fe. In the general formula (Mg,Fe,A1), [Sis_xAlxO20](OH)~6, x varies between 1.5 and 2.6; Fe/(Fe + Mg) between 0.47 and 0.83 and n between 10.80 and 11.54. Octahedral AI is close to 3 in this formulation and Fe 2 § predominates over Fe 3+. Swelling chlorites have significantly different compositions which are consistent with smectite/chlorite interstratifications.The Ib (fl = 90 ~ ) polytype appears to be stable under conditions of moderate to deep burial. It replaces berthierine and swelling chlorites formed at lower temperatures. As commonly seen in grain coatings, however, it precipitates from porewater; solutes probably being contributed from several mineral decomposition reactions.

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 FeIII 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 FeIII 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.

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