- © The Mineralogical Society Of America
Semiconducting minerals are a unique class of materials that have a broad range of influence in near-surface geochemical environments. Many are electroactive and play an important role as natural solid phase electron sources and sinks. This property gives them the capacity to control the availability of electrons in the environment, with direct and indirect effects on many redox-based geochemical and biogeochemical processes. Examples include the natural cycling of redox-active elements (Calmano et al. 1994; Nimick and Moore 1994), dissimilatory metal reduction in microbial respiration (Lovley and Phillips 1988; Lovley et al. 1991), the oxidative degradation of organic pollutants (Schwarzenbach and Gschwend 1990; Matheson and Tratnyek 1994), and precious metal ore deposit formation (Nash et al. 1981; Bakken et al. 1989). The surfaces of semiconducting minerals are therefore, in a sense, an important threshold for the natural flux of electrons in the subsurface. The transfer of electrons across the boundary occurs in the form of redox reactions at the surface, and these are influenced by specific behavior at the mineral surface. Aside from this important characteristic, this class of minerals also presents chemically distinct surfaces to the environment that can uniquely influence non-redox based processes. For instance, sites in the structures of these materials are compatible with the incorporation of many types of heavy metals (Manceau and Combes 1988; Ford et al. 1997). Their surfaces can also catalyze organic decomposition reactions (Bell et al. 1994).
The significance of semiconducting mineral surfaces in natural settings, as well as in technological applications, have generated a great deal of cross-disciplinary interest in understanding their physical and chemical properties at a fundamental level. Research with the goal of determining the structure of these surfaces and the physical basis underlying the way in which they react is a vast and …