Mn-Fe Coatings on Saprolite Fracture Surfaces

Abstract

ABSTRACT Black coated fracture surfaces are common in the saprolites of the Southeast United States. Many of the surfaces are polished and striated indicating movement has occurred; others show no evidence of movement. The zone is commonly 1-2 mm thick. The surface skin of the zone, less than 1 µm thick, commonly consists of well oriented thin clay and biotite flakes coated with amorphous hydrous Mn-Fe oxides. The underlying material is not oriented parallel to the fracture surface. The clay is an iron-rich tabular halloysite which formed from biotite. The Mn occurs as 0.1 µm ovoids thin coatings and micro-nodules. Some surfaces contain a high concentration of rare earth elements, particularly cerium. Both the Mn and Fe were released from the biotite during acid weathering (saprolite pH 5). Much of the Fe was precipitated in the vicinity of the biotite. The more mobile Mn migrated to pre-existing fractures and voids where the pH of circulating ground waters was sufficiently high (pH 7) to cause precipitation. All features indicate movement along the fractures has been extremely gentle and is probably due to settling or stress release rather than tectonic activity. The black coating has intrigued me since I moved to the S.E.. I am grateful to the Duke Power Company for supporting much of this study, C. H. Gardner of Law Engineering Testing Company aided in the sample collecting and was most helpful in defining the problem. Dr. K. C. Beck and G. A. Cooke kindly provided the chemical analyses.