BANDED
IRON FORMATION
Banded iron
formations, or BIFs, are extinct, marine sedimentary rocks (most have been
well metamorphosed) that usually consist of alternating reddish- and
silvery-gray iron-rich layers. They are most common in the
Paleoproterozoic rock record (2.5 to 1.6 Ga). They represent a time when
EarthÕs oceans Òrusted outÓ as small amounts of atmospheric free oxygen (O2
gas) combined with dissolved iron in seawater to precipitate as iron oxide
minerals. Some workers hypothesize that bacterial mats on the seafloor
mediated the precipitation of iron oxides.
Many specific varities of BIFs exist, including
jaspilite, taconite, quartzite-specularites, magnetitites, etc.
Hollywood Granite - jaspilite meta-BIF (locally called itabirite) from
the Paleoproterozoic-aged Minas Supergroup in the Iron Quadrangle District in
Minas Gerais State, Brazil. BIFs in the Iron Quadrangle have long been
mined as a source of iron ore. Recently, BIFs there have been quarried as
a source of attractive, high-priced, and very-heavy-for-their-size decorative stones.