Abstract
Transgression by the Western Interior Sea during the Late Cretaceous in southwestern Minnesota caused swampy conditions to be imposed upon a laterite consisting of gibbsite, goethite and kaolinite. Reducing conditions overprinted upon the laterite reduced ferric Fe in goethite for incorporation of ferrous Fe into Fe-berthierine. Attendant oxidation of organic matter provided CO2 for siderite’s formation. Thermodynamic calculations indicate that berthierine, gibbsite, goethite, kaolinite and siderite were in equilibrium with a solution whose pH was 5.2 and whose pCO2 was on the order of 0.3 atm. Formation of Fe-berthierine is favored by solutions having: 1) low silica concentration; 2) low [Mg2+]/[Fe2+] ratio; 3) high pCO2; 4) extremely low sulfate content before reduction takes place; and 5) moderate reducing conditions (Eh around −0.05 V).
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Fritz, S.J., Toth, T.A. An Fe-Berthierine from a Cretaceous Laterite: Part II. Estimation of Eh, pH and pCO2 Conditions of Formation. Clays Clay Miner. 45, 580–586 (1997). https://doi.org/10.1346/CCMN.1997.0450409
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DOI: https://doi.org/10.1346/CCMN.1997.0450409