Surface hydroxyl configuration of various crystal faces of hematite and goethite

The surface properties of oxides and hydroxides depend on their surface hydroxyl configuration (SHC). Unfortunately, the SHC of the various Fe (hydr)oxides has not been systematically studied to date. This paper reports the SHC of the hematite {100}, {110}, {012}, {104}, {018}, {113}, and {001}, and the goethite {100}, {010}, {110}, and {021} faces, which are the most common in natural and synthetic specimens. Both minerals exhibit marked differences between faces, particularly with regard to the presence and proportion of singly, doubly, and triply coordinated OH groups. Pairs of contiguous singly coordinated OH groups, which are involved in the specific adsorption of phosphate and other ions, occur on all the goethite faces studied, but not on those of hematite. According to the hypotheses inherent in some models, the variable SHC of hematite and goethite faces should result in marked differences in proton adsorption and, hence, surface charging properties. (C) 1996 Academic Press, Inc.