Banded iron formations (BIFs) are sedimentary rocks, characterized by the presence of alternating layers of iron-rich minerals - known as magnetite - and amorphous silica-rich rocks - known as cherts. Each layer is relatively thin with varying levels of thickness, measuring from a millimeter up to several centimeters. The phrase ‘banded iron formation’ resulted from the characteristic color banding of these rocks.
BIFs occurred in Proterozoic rocks about 2.5 billion years ago in the Archean Eon, during a time period when the atmosphere and ocean had no oxygen. Geologists consider that BIFs provide the crucial insights regarding the evolution of the early earth. Most iron ore deposits in the world are located in BIFs.
Almost all BIF formations have experienced a certain amount of faulting, fracturing, metamorphism, compaction, veining, folding, and intrusions. Many specific varieties of BIFs have been identified. One such example is jaspilite, which is a reddish, silvery gray banded rock consisting of hematite, red chert (referred to as jasper), and hematite or magnetite. When polished, BIFs are very attractive.
Apart from the mining of iron, BIFs are used for ornamental purposes, such as bookends, clock faces, and in monuments.
Extensive BIF deposits have been found in Western Australia, and Minnesota, USA.