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

Steven J. Fritz and Thomas A. Toth
Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907

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).

Key Words: Berthierine • Gibbsite • Goethite • Cretaceous • Kaolinite • pCO2 • pH • Siderite • Soil Zone • Weathering

Clays and Clay Minerals; August 1997 v. 45; no. 4; p. 580-586; DOI: 10.1346/CCMN.1997.0450409
© 1997, The Clay Minerals Society
Clay Minerals Society (www.clays.org)