Rock Specimens on the Lawn, Banded Iron Formation, 1100 North University Building, 1100 N University Ave, University of Michigan, September 22, 2020

https://lsa.umich.edu/earth/about-us/about-our-building/rock-specimens-on-the-lawn.html

Banded Iron Formation (BIF)

'Early Proterozoic probably from near Marquette in the Upper Peninsula of Michigan; red quartz layers and black layers of specular hematite and magnetite. The layering is though to be produced biannually in shallow seas. In general, banded iron formations of the oxide-carbonate-silicate- sulphide type occur in Archean greenstone belts and reached their peak development in the early Proterozoic basins about 2000-1800 Ma ago. Mid-late Proterozoic and Phanerozoic basins do not contain BIF of this type. In contrast, iron formations containing chamosite-goethite-siderite are unknown in the Precambrian but are found in Phanerozoic basins. The occurrence of the Precambrian BIF's is thought to be related to anoxic conditions of the early atmosphere. As oxygen content of the atmosphere and ocean waters increased, the solubility of iron decreased. With these changes in solubility, transport of iron in the entire weathering cycle would have decreased. Iron would remain in soils and not be found in solution in ocean waters after the atmosphere became oxygen-rich.'