Iron and Oxygen: a Dangerous Mixture

During the chemical and biological evolution of the Earth, its atmosphere changed from a highly reducing state to the oxygen-rich state that we know today. Free molecular oxygen (dioxygen) probably appeared on the Earth's surface some 2 billion years ago as a result of photosynthetic microorganisms acquiring the ability to split water. Oxygen is now the most abundant element in the Earth's crust and the second most abundant element in the biosphere. The concentration of molecular oxygen (hereafter referred to as oxygen) in dry air has risen to 21%. Iron, the element of equal importance in this story, is the fourth most abundant element in the Earth's crust.