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 Earth’s oceans “rusted
out” 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.