Posidonian schists with imprinted ammonites acquired a golden hue due to “silly gold”. Reported by the University of Texas at Austin.
The Posidonian Shale is a fossil found in Germany and formed in the early Jurassic period 183 million years ago. Many have stigmatized living organisms, including ammonite mollusks and squid. At the same time, some of the ammonites have a distinctive golden color associated with the pyrite (iron disulfide) mineral, formerly “fool’s gold.”
Rowan Martindale and colleagues analyzed them and found that this was not the case. The researchers found that the fossils are mostly composed of phosphate minerals, but the surrounding black shale rock is filled with microscopic deposits of pyrite crystals called framboids.
“For some specimens, I counted 800 framboids in the surrounding rock, while in the fossils there were three or four,” said one of the authors.
The presence of pyrite and phosphate in different locations in the samples is important in terms of revealing important details about the fossil environment. Pyrite forms in an oxygen-free environment, but phosphate minerals need oxygen. The study suggests that while the anoxic seafloor protects against decay, it forms the basis of fossilization, while oxygen is needed to drive the chemical reactions necessary for fossilization.
“For a long time it was believed that the absence of oxygen contributes to exceptional protection, but this does not directly help. This helps create an environment conducive to faster petrification, which results in protection [отпечатка живого организма]but it is oxygen saturation that improves protection,” conclude the scientists.
It was the phosphates and related minerals that increased the brightness of the fossil.