Magnetite, the mineral that taught people magnetism, also sits in brains
A chunk of naturally magnetised magnetite, called lodestone, will tug small bits of iron toward it, and that is how ancient people first stumbled on magnetism. The same black iron oxide steers bacteria, sits in the beaks of migrating birds, coats the teeth of a mollusc and turns up scattered through the human brain.
Chemically it is iron(II,III) oxide, holding both ferrous and ferric iron, and apart from rare native iron it is the most magnetic mineral found naturally on Earth. It is black with a metallic sheen, scores 5 to 6 on the Mohs scale and leaves a black streak. Its structure, worked out in 1915, was among the first ever solved by X-ray diffraction. Heat it past 580 °C and it loses its magnetism; cool it to about 120 K and its crystal structure abruptly shifts, a change called the Verwey transition.
Lodestones served as early compasses, and magnetite grains that lock in the direction of Earth's field made paleomagnetism, and with it plate tectonics, possible to study. Geologists also use its reactions with oxygen, forming hematite, to gauge how oxidising ancient magmas were. Big enough deposits can throw compasses off; parts of Tasmania demand repeated checks. Mauritania's highest peak, Kediet ej Jill, is made entirely of it, and black beach sands in California and New Zealand owe their colour to it. In 2005 a company found Peruvian dunes covering 250 square kilometres whose sand is 10 per cent magnetite.
Living things make it too. Magnetotactic bacteria build chains of magnetite particles to navigate, and those chains can survive as magnetofossils. Some birds carry crystals in the upper beak that, alongside eye pigments, let them sense magnetic fields, and chitons coat their rasping teeth with it for extra hardness.
Human brains contain it in many regions, including the hippocampus. Microscopes can tell the body's own jagged crystals from rounded nanoparticles made by combustion, which can enter the brain through the olfactory nerve. In some samples the pollution particles outnumbered natural ones by as much as 100 to 1, a possible link to abnormal neural decline.
Source: Magnetite
