An Fe-Berthierine from a Cretaceous Laterite: Part I. Characterization

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

Abstract: An Fe-berthierine occurs in a buried laterite from the Late Cretaceous (Cenomanian) in southwestern Minnesota. It formed beneath a lignitic horizon in which reducing solutions percolated through a laterite comprising gibbsite, kaolinite and goethite. Morphologic differences suggest 2 separate conditions of Fe-berthierine formation. Early forms of Fe-berthierine include radial bladed or radial blocky crystallites coating pisoids, along with alteration of kaolinite at crystal boundaries. These morphologies formed in the vadose zone. Later forms precipitated under subaqueous conditions as macroscopic, pore-filling cement. The large size of the later-formed Fe-berthierines enabled microprobe characterization. This 1st reported occurrence of Mg-free berthierine has a structural formula close to an idealized Fe-berthierine: Fe2Al2SiO5(OH)4. Apart from their chemistry, the unique feature of the Minnesota Fe-berthierines is their formation in an exclusive nonmarine depositional environment. They formed in situ as part of a lateritic weathering profile developed on a broad, low relief peneplain. Physical evidence of formation under nonmarine conditions includes the presence of 1) scattered lignitic fragments; 2) concretions forming casts and molds of woody material; and 3) a nonmarine fossil (Unio sp. undet). Chemical evidence includes siderites collected from the berthierine-bearing horizon having stable isotope values indicating freshwater formation.

Key Words: Berthierine • Cretaceous Weathering • Kaolinite • Minnesota • Pisoid

Clays and Clay Minerals; August 1997 v. 45; no. 4; p. 564-579; DOI: 10.1346/CCMN.1997.0450408
© 1997, The Clay Minerals Society
Clay Minerals Society (www.clays.org)