AUTHIGENIC CHLORITES: PROBLEMS WITH CHEMICAL ANALYS IS AND STRUCTURAL FORMULA CALCULAT ION

A B S T RA C T: Analytical transmission electron microscopy was applied to some authigenic chlorites occurring as grain coatings in sandstones. Compositional variation proved to be relatively slight: all were magnesian chamosites. The coating chlorites were often intimately mixed with extremely fine-grained (0.01-0.2 #m) hematite but analytical 'contamination ' was avoided because of the very high resolution of both observation (spot location) and analysis. One example of a water sensitive ('swelling chlorite') coating was also studied. This proved to have a very much more variable composition even within a single section. The coating appeared to include both chloritic and vermiculitic components. The effect of this on structural formulae is discussed and a model proposed inwhich the 'talc ' layer may be common to both components. Chlorite is found as an authigenic mineral in many sandstones. It usually occurs as grain coatings, less commonly filling pores. It has been suggested that these coatings can be so continuous that they prevent overgrowth of framework grains from saturated porewaters, thereby preserving porosity. The chlorite structure consists of a 2:1 layer (talc-like) plus an interlayer hydroxide sheet (Bailey, 1980) which in sedimentary examples is iron-rich. This is readily attacke