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.




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