Manganese oxides and hydroxides are found in many geological settings and occur in the form of more than 30 minerals. 1 They participate in a variety of redox and acid/base reactions in the environment because they form interfacial barriers on soils and sediments in marine and freshwater environments and on rock and other mineral surfaces. The ability of manganese oxides to adsorb and oxidize heavy metal pollutants, such as chromium and arsenic, has been recognized for decades. Because of their ubiquity in the environment and their reactivity, these minerals serve as an important control in regulating the concentrations of groundwater contaminants. Manganese oxides are also commercially relevant as the cathode material in alkaline batteries 2,3 and have been studied for use in catalytic applications 4À6 and water treatment media. 7,8 Their redox behavior indicates that manganese oxides may potentially act as sensors as well. 9 Despite the importance of manganese oxides in geochemical processes and in technological applications, their surface structures and properties have not been well studied. For example, recent reports indicate that manganese oxides can oxidize Cr III to toxic Cr VI10À17 and toxic As III to As V . 18À22 Although these studies aimed at understanding the role of manganese oxides in the oxidation of heavy metals, it remains unclear which manganese species participate in the process and which manganese oxide surfaces are relevant for redox chemistry. A fundamental knowledge of the surface structures and their redox properties at environmentally relevant conditions is the necessary groundwork to be laid before complex interactions between heavy metal ions and manganese oxide surfaces can be fully delineated.A small number of studies have examined manganese oxide surfaces. The earliest work demonstrated reduction 23 and oxidation 24 of manganese oxide surfaces using X-ray photoelectron spectroscopy (XPS). The surface structures, however, were not characterized in either study. More recently, Langell et al. 25 examined the MnO (100) surface using high-resolution electron energy loss spectroscopy (EELS) and showed that oxidation of the surface leads to the formation of Mn 2 O 3 and subsequently Mn 3 O 4 . Intermixture of the oxide species prevented isolation of pure-phase layers and hence structural determination of the surface. EELS in scanning transmission electron microscopy was also used to analyze manganese valence at the surfaces of natural manganese oxides. 26 It was found that the average valence at the β-MnO 2 surface was 4.0, in agreement with the formal ABSTRACT: Stoichiometric and defective terminations of the β-MnO 2 (110), (100), and (101) surfaces are investigated as a function of oxygen partial pressure and temperature using ab initio thermodynamics. In agreement with studies on other rutile-type minerals, the (110) surface is predicted to be the most stable surface, followed by the (100) surface and then the (101) surface. The (110) and (101) surfaces are found to oxidize by formation...