BANDED IRON FORMATION
Banded iron formations, or BIFs, are iron-rich, marine sedimentary deposits that generally consist of alternating bands of reddish & dark-colored minerals. The red layers are typically red chert ("jasper" - cryptocrystalline quartz (SiO2) stained red by hematite) and/or hematite (Fe2O3 - iron oxide). The dark layers are typically very dark gray to silvery gray in color and are composed of magnetite (Fe3O4 - iron oxide) and/or specular hematite (micaceous hematite; Fe2O3). The specular hematite component of BIFS usually formed by metamorphism (most BIFs have been slightly to significantly metamorphosed).
BIFs vary in their appearance, mineral compositional details, and coloration. The BIF shown below is jaspilite. Other BIF rock types include taconite, slaty taconite, cherty taconite, magnetitite, specularite, quartzitic specularite, specularitic quartzite, etc.
The origin of BIFs is not well understood, principally because they do not form today (they're "extinct" rocks). Most BIFs are in Paleoproterozoic-aged successions. The first ones are late Archean in age (>2.5 billion years), but most range in age from 2.5 to 1.8 billion years, when Earth’s atmosphere had very little free oxygen gas (O2). What little oxygen gas was present then reacted with iron dissolved in seawater, resulting in the precipitation of BIF layers on ancient seafloors.
Banded iron formation (BIF) (cut & polished surface)