Banded iron formation

Article By:

Trendall, Alec Francis Department of Applied Physics, Curtin University of Technology, Perth, Western Australia.

Last reviewed:October 2019

DOI:https://doi.org/10.1036/1097-8542.757494

A sedimentary rock that was commonly deposited during the Precambrian. It was probably laid down as a colloidal iron-rich chemical precipitate, but in its present compacted form it consists typically of equal proportions of iron oxides (hematite or magnetite) and silica in the finely crystalline form of quartz known as chert. Its chemical composition is 50% silicon dioxide (SiO₂) and 50% iron oxides (Fe₂O₃ and Fe₃O₄), to give a total iron content of about 30%. Banding is produced by the concentration of these two chemical components into layers about 1–5 cm (1/2–2 in.) thick; typical banded iron formation consists of pale silica-rich cherty bands alternating with black to dark red iron-rich bands (Fig. 1). These contrasting layers are sharply defined, so that the rock has a striped appearance; banded iron formation is normally a hard, tough rock, highly resistant both to erosion and to breaking with a hammer.

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