Glauconite has been identified from 6 localities in the Precambrian Biwabic Fe-formation of the Mesabi range. The appearance in thin section is that of a normal diagenetic alteration. The field occurrence suggests a secondary origin: 1) localization of the glauconite in the border zone between Fe ore bodies and unoxidized Fe-formation, 2) occurrence at different horizons within the Fe formation from one mine to the next, and 3) as cross-cutting fracture-fillings and in quartz and chalcedony veinlets. Confirmation of a secondary origin is given by K-Ar dates of 100 and 150 million years for 2 specimens. It is suggested that the glauconite formed when the Cretaceous sea covered the Precambrian terrane. The porous Fe ore bodies at the surface were saturated with marine waters that reacted with unoxidized to partially oxidized Fe formation in the border zones and along fractures to form glauconite of Cretaceous age in a rock of Precambrian age. Glauconite-bearing Cretaceous sediments lie directly upon the Biwabic Fe formation at several localities. Failure of glauconite to form in Precambrian time is attributed either to absence of the proper salinity level or to lack of a favorable environmental oxidation potential. The Mg contents of 3 analyzed specimens are comparable to that of the average marine glauconite, as determined from 52 analyses from the literature. The amount of tetrahedral substitution in 2 specimens is normal for marine glauconites but for a third specimen is lower, approaching that for nonmarine occurrences. The Fe-rich environment is reflected in unusually high ferric and ferrous iron contents, whereas the octahedral Al values and ferric/ferrous ratios are unusually low. A trend is noted towards increase in octahedral cation total with decrease in interlayer cation total.