New acquisition: A rock that is 2.5 billion years old

A spectacular specimen has been acquired for the AM’s upcoming Minerals exhibition. The geological story the rock tells and its striking appearance will offer visitors a window into the deep history of our planet.

This extraordinary polished slab, measuring 2m by 1.5m and weighing 437kg, is part of the Archean Nimingarra Formation in the Pilbara, Western Australia. It shows us a dramatic change in the Earth’s past atmospheric and biological conditions, frozen in time in its dramatic layers: the oxygenation of the oceans.

Banded Iron Formation (BIF) is the name given to these finely striped and kink-folded, alternating layers of black 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 an ancient sea floor some 2.5 billion years ago.

Their deposition resulted from a dramatic change in the 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. These became insoluble as the iron combined with the oxygen, and iron oxides and silica precipitated. This made iron and silica-rich sediments, which accumulated in alternating bands on the ocean floor. This cyclical process continued for nearly a billion years. Heat and pressure from the Earth’s movements dramatically folded and kinked these rocks.

This acquisition was funded by a grant from the Patricia Porritt Collection Acquisition Fund and the Australian Museum Foundation. The BIF will be on display in the upcoming Minerals exhibition, due to open December 2022.

This article was originally printed in Explore, the Australian Museum Members' magazine. Read the issue.