Abstract

Available evidence regarding occurrences of magnetite believed to be of supergene origin is assembled, and the conclusion drawn that they are reasonably common, although economically unimportant. Field occurrences and laboratory data suggest that such magnetite is formed only below the water table, or in a water basin, where a delicate and usually unstable balance between oxidizing and reducing tendencies exists and a suitable precipitant, such as calcium carbonate or a hydroxide, is present. The resulting magnetite generally is very fine-grained, and usually is associated with much larger amounts of soft earthy hematite, and minor quantities of minute specularite grains. Ringed alternations of magnetite and hematite, as in oolites or centripetal replacements, are very common.

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