Fossilized and opalized brachiopod

Brachiopods are marine animals with bivalve calcareous shells(the shells of which are made up of two valves). These specimens are 182 million years old(Toarcian, Lower Jurassic).

Opalisation process of a shell at Coober Pedy in Australia:

About 150 million years ago the closed shell buries itself in the clay sediments. Then, the pressure increases with the sediments which accumulate on top(the shell does not contain sediments). 30 to 90 million years ago the carbonates dissolve and the increase in pressure leads to an enrichment in silica thanks to the underground waters rich in dissolved silica. About 20 to 30 million years ago, variations in the water table produced a deposit of Opal while the shell was covered by silica-saturated liquids. The process of opalization/silicification is rather recent and rapid(only a few decades, according to the experiments of Eckert, 1977).

The Coober Pedy Opals were discovered in the 1915s during the Australian Gold Rush. They are prospected at depth in the layers of sandy clays deposited by the shallow sea that covered the territory 100 million years ago(Cretaceous era) .

Source www.futura-sciences.com