Abstract The interaction between silica, manganese, clay minerals and carbonate was investigated experimentally at 25°C and 70°C using commercial silica gel, manganese monohydrosulfate, smectite, aragonite oolites and calcite foraminifera. The calcium carbonate polymorph, aragonite, altered to manganese carbonate within 6 months at both 25°C and 70°C. Calcite conversion to manganese carbonate occurred at 70°C but not at 25°C over the same time period, and much less rapidly than aragonite even at the higher temperature. Excess calcium released into solution by the carbonate conversion precipitated as gypsum. The mechanism for conversion of calcium carbonate to manganese carbonate appears to consist of initial sorption of manganese onto the surface of calcium carbonate phases, followed by dissolution of these phases and subsequent precipitation of manganese carbonate. The manganese oxyhydroxides, hausmannite and manganite, formed at 70°C within 6 months apparently by direct nucleation on smectite and through oxidation of previously formed manganese carbonate. At 25°C, only trace amounts of hausmannite were detected after 6 months, and no manganite was found. These results follow the logical progression of phase stability in oxygenated aqueous systems predicted from Eh—pH diagrams for the manganese system, where manganese carbonate is unstable relative to hausmannite, which is in turn unstable relative to manganite.