File:Interbedded specularite-quartzite (meta-BIF) (Negaunee Iron-Formation, Paleoproterozoic, 1.874 or 2.11 Ga; Republic Mine, Marquette County, Upper Peninsula of Michigan, USA) 2.jpg

English: Interbedded specularite-quartzite from the Precambrian of Michigan, USA. (7.6 centimeters across at its widest)

Silvery-gray = specularite, composed of specular hematite (micaceous hematite) (Fe2O3) Dark purplish-gray = quartzite (SiO2)

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.

A famous BIF unit is the 1.874 or 2.11 billion year old Negaunee Iron-Formation, which outcrops in the Marquette Iron Range of Michigan’s Upper Peninsula (UP), USA. The Negaunee Fe-Fm. has been metamorphosed to varying degrees and contains many specific lithologies, including jaspilites, taconites, specularites, ferruginous quartzites, ferruginous cherts, and ferruginous slates. The Negaunee Fe-Fm. has economic concentrations of iron and much of the unit in the Marquette Iron Range has been mined away. The best remaining, easily-accessible outcrop is Jasper Knob in the town of Ishpeming, Michigan. Earth’s oldest known macrofossils, Grypania spiralis, occur in this unit.

The specimen seen here is a metamorphosed BIF - the dark purplish gray layers are quartzite, metamorphosed from chert. The silvery-gray layers are specular hematite (micaceous hematite), which has been metamorphosed from "ordinary" rusty red hematite.

Stratigraphy: Negaunee Iron-Formation, Paleoproterozoic, 1.874 or 2.11 Ga

Locality: Republic Mine, western Marquette County, Upper Peninsula of Michigan, USA