1900 – 1800 million years ago

Year twenty-eight
Uraninite has black, dark grey or dark brown cubic crystals. It is sometimes known by the name 'pitchblende'. It is shown here within an igneous rock.

Detrital uraninite disappears

Uraninite is a mineral composed of one uranium atom for every two oxygen atoms (it is given the chemical formula UO2). It is usually found in igneous rocks that formed deep inside the Earth. When it is in our oxygen-rich atmosphere it changes its chemical formula so that for every three uranium atoms there are eight oxygen atoms (the chemical formula U3O8). This changes the way uraninite behaves, because U3O8 dissolves in water.

In Precambrian rocks older than about 1900 million years of age, uraninite can be found in ancient sediments that were laid down on the beds of streams and rivers. This 'detrital' uraninite must have had the formula UO2 because if it had been U3O8 it would have dissolved in the stream water. As the UO2-type uraninite can only exist in the absence of oxygen, its presence in the stream sediment proves there was no oxygen in the Earth's atmosphere at that time.

Oxygen was being given off by photosynthesising organisms. First it was trapped in the sediments on the ocean floor; later it was trapped by turning iron in sands on the land into rust forming the red beds, but finally it began to enter the atmosphere. At first it occurred in small amounts, but it gradually began to build up until 1900 million years ago there was so much in the atmosphere that uraninite was not able to form in stream sediments — it just dissolved.


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