Taconite in the Precambrian of Minnesota, USA.

Banded iron formations, or BIFs, are unusual, dense, marine 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. They do not form in today's oceans - they're an example of an “extinct” rock type. Many specific varieties of iron formation are known. The most attractive type is jaspilite, which is a reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite. Most BIFs have been subjected to one or more orogenic (mountain-building) events, resulting in folding and/or metamorphism. BIFs are known from around the world, but some of the most famous & extensive deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined as iron ores.

The above photo shows an outcrop of taconite in the Mesabi Iron Range of Minnesota. Taconite is a low-grade iron ore lithology that is a mix of iron oxide and silicate minerals. The Mesabi Iron Range was mined starting in the 1800s. The original target rocks were high-grade iron ores rich in hematite and low in silicates. Modern mining in this area targets taconite, which has a relatively high silicate content.

The brick red coloration is a surficial coating of hematite (Fe2O3 - iron oxide), produced by weathering, that has covered the taconite. Fresh taconite surfaces here are dark gray.

Stratigraphy: Biwabik Iron-Formation, Paleoproterozoic, ~1.878 Ga