Abstract~Chlorite and illite are the major clay minerals in silicate assemblages from a rock salt bed in the Vernon Formation (Upper Silurian) at Retsof, New York. Textural features and Br content of the salt indicate precipitation from shallow marine brine with no subsequent postdepositional recrystallization. Sample mounting procedure for electron microprobe analysis involves clay particle dispersion, sedimentation, and transferral to a planar silver print surface. The 001 face of the flake, rather than the conventional polished plane, constitutes the analyzed surface. with little variation among samples. The illite is distinctly less degraded than normal shale illite suggesting some recrystallization occurred in the hypersaline environment. Extensive compositional variation among illite flakes within each sample may reflect alteration of several different detrital micaceous minerals. The term hyperhalmyrolysis is introduced to denote mineral reactions which occur in the marine hypersaline environment.Diagenesis effected improved crystallinity and undoubtedly involved isochemical recrystallization of the bulk silicate assemblage. Metasomatism of the assemblage during diagenesis, however, appears to be negligible.

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