An Fe-Berthierine from a Cretaceous Laterite: Part II. Estimation of Eh, pH and pCO₂ Conditions of Formation

Abstract

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 CO 2 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 pCO 2 was on the order of 0.3 atm. Formation of Fe-berthierine is favored by solutions having: 1) low silica concentration; 2) low [Mg 2+ ]/[Fe 2+ ] ratio; 3) high pCO 2 ; 4) extremely low sulfate content before reduction takes place; and 5) moderate reducing conditions (Eh around −0.05 V).