Banded iron formation from the Precambrian of Australia. (11.5 cm across at its widest)

Orangish-brown = quartz mixed with limonite Silvery-gray = hematite

Banded iron formations, or BIFs, are unusual, dense sedimentary rocks consisting of alternating layers of iron-rich oxides and iron-rich silicates. Most BIFs are Proterozoic in age (although some are Late Archean), and do not form today - they're “extinct”! Many specific varieties of iron formation are known, and some are given special rock names. For example, jaspilite is an attractive reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite.

Because of their age, most BIFs have been around long enough to have been subjected to one or more orogenic (mountain-building) events. As such, most BIFs are folded and/or metamorphosed to varying degrees.

BIFs are known from around the world, but some of the most famous & extensive BIF deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined. BIFs are the most important variety of iron ore on Earth.

The BIF sample shown above is from South Australia’s Lower Middleback Fe-Fm. (a.k.a. Lower Middleback Jaspilite). This rock is dominantly composed of silvery-gray hematite and yellowish-brown limonite (FeO(OH)·nH2O). Portions of the Lower Middleback Fe-Fm. have economic concentrations of iron & are actively mined.

Stratigraphy: Lower Middleback Iron-Formation, Middleback Subgroup, Hutchinson Group, upper Paleoproterozoic, 1.81 Ga or ~1.859-1.945 Ga