Banded iron-formation (BIF), illustration. BIFs are sedimentary rock formations with alternating silica-rich layers and iron-rich layers that are typically composed of iron oxides (hematite and magnetite), iron-rich carbonates (siderite and ankerite), and/or iron-rich silicates. They are typically several metres to several hundred metres in thickness, and extend laterally from a few kilometres to several hundred kilometres. They were deposited in Pre Cambrian times, around 2 billion years ago, during a phase in the evolution of the earth known as 'the great oxygen event'.