Despite their ubiquity and high visibility, rust colored iron oxide deposits remain a poorly understood feature of rivers, lakes, and springs. Until recently, it was assumed that these deposits were formed by abiotic chemical processes; however, recent studies have shown that these deposits often result from the action of specific iron oxidizing bacteria. These bacterial iron oxides may have very different properties than those abiotic oxides that are traditionally used by geochemists in laboratory experiments designed to probe environmental and geochemical reactivity. The overall goal of the project is to characterize the structure, reactivity, and biological diversity of these iron oxide deposits to better understand the multifaceted roles they may play in the environment. To achieve these goals, we will conduct a suite of field, laboratory, microbiological, and spectroscopic analyses aimed at determining key properties of both the iron oxides and the bacteria associated with their formation.