Every Rock Has A Story - Banded Iron

Banded iron is a rock comprised of alternating layers of red silica (jasper) and iron oxide (hematite or magnetite) – this 1.8 to 2 Byr old specimen comes from the Marquette Range of the Upper Peninsula and is a variety known as Jaspelite – the silver/gray iron layers are specular hematite. Banded iron is the main source of iron ore on Earth. Globally the bulk of the Lake Superior-type banded iron was chemically precipitated from the sea between 1.8 Byr to 2.5 Byr. It is evidence that the world’s oceans lacked free oxygen for much of their early history – allowing ferrous iron to accumulate in solution in vast quantities – the iron was eventually oxidized and precipitated as ferric iron by several possible mechanisms including: chemical oxidation by anaerobic bacteria; the spread of photosynthetic oxygen in the oceans from cyanobacteria; and the formation of free oxygen liberated from water by UV radiation.