Iron oxide

Iron oxide is iron chemically bonded to oxygen. You know it as rust, but it's also the rock nearly all iron comes from. The two most important forms are hematite, Fe₂O₃, which is reddish, and magnetite, Fe₃O₄, which is black and naturally magnetic.

Iron and oxygen combine readily. Given air and water, iron metal slowly turns back into oxide; that's rusting. Over geological time, the same chemistry built huge ore deposits. Many of the richest formed roughly 2.5 to 1.8 billion years ago, when oxygen made by early microbes reacted with iron dissolved in the oceans, and the oxide settled in banded layers on the seafloor. Today the ore is mined in open pits, crushed and often formed into pellets. For electronics, a much purer iron oxide is made chemically, for example by roasting iron chloride left over from cleaning steel with acid.

Ore goes into a blast furnace to become pig iron, the first step toward steel. Purified Fe₂O₃ makes up most of ferrite, the magnetic ceramic in inductors and transformers. Iron oxides are also among the oldest pigments: the red and yellow ochres of cave paintings.