Banded iron formations (BIFs) are iron oxide- and silica-rich chemical sedimentary rocks that were deposited in great abundance during an approximately 800-million-year span from the late Archean to the mid-Proterozoic eons (circa 2.7–1.9 billion years ago). At that time, the terrestrial atmosphere was changing in composition from early anoxic conditions to a more oxygenated state, due possibly to the increasing activity of photosynthetic organisms. It has been suggested that Mars and Earth were more similar in their early history than they are today, and that analogous geologic processes—including those responsible for BIF deposition—may have operated on both planets. Here we discuss BIF samples collected from drill cores at Carajas, Brazil, that represent a variety of fresh, weathered, and altered rocks. Laboratory and remote sensing measurements of these rocks show unique properties that may assist in the discovery of any similar deposits on Mars.