Banded Iron Formation

Weighing a whopping 437 kg, this spectacular specimen is part of the Archean Nimingarra Formation of Western Australia and illustrates a dramatic change in Earth’s past atmospheric conditions.

Banded Iron Formation, or BIF, is the name given to these banded and kink-folded alternating layers of iron oxide (hematite and magnetite) and microcrystalline silica (red and brown chert and golden-brown tiger eye). They used to be iron-rich and silica-rich sediments on the ancient sea floor about 2.5 billion years ago.

Their deposition resulted from a dramatic change in Earth’s atmosphere, when the first micro-organisms that could produce oxygen by photosynthesis (cyanobacteria) greatly increased the oxygen content of oceans and the atmosphere. The iron and silicon from rock weathering and ocean floor volcanic hot springs dissolved in seawater. The dissolved iron and silicon combined with oxygen in the water to make insoluble iron oxides and silica which came out of the solution as a fine-grained sediment. This made iron oxide- and silica-rich sediments accumulate in alternating bands on the ocean floor. This repeated process continued for nearly a billion years. Heat and pressure from Earth’s movements dramatically folded and kinked these rocks.

Purchased by a grant from the PP Collection Acquisition Fund and the Australian Museum Foundation.