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)