Rocks Conduct Chemically

To determine the efficiency of treatments that remove iron(III) oxide rust, researchers typically measure the amount of water-soluble iron(II) that the treatments release. That kind of measurement doesn't tell the whole story, say Kevin M. Rosso and Svetlana V. Yanina of Pacific Northwest National Laboratory. They show that hematite, a naturally semiconducting iron(III) oxide, conducts electricity under certain chemical conditions without voltage application, thus enabling some soluble iron(II) to be recycled as iron(III) deposits (Science, DOI: 10.1126/science.1154833). Hematite's different crystal surfaces have varied microstructures, each with different chemical functionalities that dictate whether it can conduct electricity. The team made millimeter-sized hematite cubes with these different microstructures exposed on each face. Under reducing conditions, most microstructures dissolved away as iron(II), as expected. On one type of microstructure, however, pyramid-shaped iron(III) deposits grew. Pyramids appeared only when the arrangement of microstructures allowed electrons to flow, and the researchers conclude that the reduction process triggers current flow.