The iron silicate minerals from more than 50 specimens of the Proterozoic Gunflint Iron formation, near Port Arthur, Ontario, were identified by X-ray powder methods and microscopic techniques. There appears to be a geographical and possibly a stratigraphical distribution of the mineral species. Chamosite and chlorite characterize the lower Gunflint, which contains relatively little iron silicate; the intermediate argillite-tuff member contains a glauconite-bearing calcareous horizon, along with chamosite, chlorite, and talc; and the upper Gunflint is characterized by abundant greenalite and stilpnomelane, with minor amounts of minnesotaite and chlorite, identified optically. The ratio of stilpnomelane to greenalite appears to increase in the vicinity of the Logan Sills.

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