Pseudomorphic replacement of manganese oxides by iron oxide minerals

Abstract

Manganese oxide minerals containing structural Mn 4+ ions can act as oxidizing agents towards ferrous ions in solution. This oxidation causes the precipitation of Fe(III)-oxides in association with manganese oxide minerals, a probable mechanism for the natural formation of iron oxides in concretions and nodules in the subsurface horizons of soil, where transport of reduced iron takes place. The synthetic manganese oxide minerals todorokite and birnessite when exposed to ferrous ions caused a pseudomorphic precipitation of iron oxide minerals. The pseudomorphism was observed to be a macroscopic phenomenon; the iron oxide precipitate consisted of fine crystallites of feroxyhyte, lepidocrocite, or akaganeite in a loose mesh having the form of the original manganese oxide mineral. The anions present and the original structure of the manganese oxide mineral influenced the nature of the iron oxide formed. Reactions of ferrous sulfate (0.05 M) with Na-birnessite, todorokite or soil minerals containing birnessite and lithiophorite yielded sulfate-adsorbed-feroxyhyte as the major product. Reactions of ferrous perchlorate (0.05 M) with Na-birnessite yielded lepidocrocite and with todorokite yielded lepidocrocite and akaganeite. Reactions of ferrous chloride with birnessite or todorokite yielded akaganeite as the major phase. All iron oxides formed were poorly crystalline and contained adsorbed water.