Concentrations of manganese in the crust are dominated by a single ore type described as the orthoquartzite-glauconite-clay association or as "manganese giants." Current research suggests that these large accumulations of manganese oxide and/or carbonate are associated with relatively rapid marine transgressions and regressions across older basement rocks. Some of the larger deposits of this type, such as Nikopol and Chiatura, USSR, and Groote Eylandt, Australia, occur in petrologically simple terrigenous formations, generally sandstones and shales, interpreted as paleoshoreline sediments. A major problem lies in the very high Mn/Fe ratio of these deposits (30-50) as against that of ordinary sediments (0.02). Research on the Groote Eylandt Cenomanian age deposit suggests that the high Mn/Fe of the primary ore (15-20) was associated with manganese enrichment in coastal waters, which in part was related to glauconitization (iron-trapping) reactions. Mineralogical evidence indicates a sequence from a well-defined glauconite phase in deeper water, off-shore sediments (with whole-rock Mn/Fe of 0.3-1.2) to a glauconite-smectite in near-shore estuaries and rivers, as a minor component of pisolitic manganese oxides of Mn/Fe ranging from 15 to 20. Data on these glauconite minerals, including X-ray diffraction, infrared spectroscopy, microanalyzes, textural occurrences, and genesis are presented. Geologically glauconite is a product of a postoxic environment, possibly related to fluctuating water depth during transgression-regression, seaward from the site of manganese oxide precipitation.