Magnetite is an iron-oxide mineral that occurs naturally on Earth. Because it is also an important component of many anthropogenic materials (e.g., coal fly ash) and synthetic products (e.g., black toner powders), magnetite can be released to the environment through human activities (1). In PNAS, Maher et al. (2) describe the abundant presence in the human brain of magnetite nanoparticles, some of which they attribute to air pollution. This finding could have major implications. Magnetite belongs to the spinel group. It crystallizes in the cubic crystal system (Fig. 1) and can be described by the general formula Fe2+Fe3+2O4 (3). Magnetite is a common natural phase, occurring in various geological environments, ranging from igneous (e.g., layered ultrabasic rocks, basalts) to sedimentary (e.g., banded iron formations, beach sands) rocks, and to high-grade metamorphic rocks (e.g., schists, skarns), where it can be produced through a multitude of chemical reactions. Due to its tendency to react with oxygen to form hematite (Fe2O3) and various iron oxyhydroxides (e.g., ferrihydrite, goethite), magnetite can be used as a powerful tool to explore oxygen concentrations in rocks during geological processes, changes in the oxygen content of the atmosphere (e.g., early Earth), and redox conditions in near-surface environments (e.g., oxic-anoxic transition zone). Because magnetite is ferrimagnetic, it represents a phase that is essential for paleomagnetic investigations, which help in reconstructing plate tectonics through Earth’s history.
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