Pyrite was long thought to be a shiny mineral, also known as fool’s gold – was a special ingredient that gave the fossils found in the German Posidonium Shale their golden radiance. But now scientists think something else is causing these fossils to shimmer.
Earlier this year, a team of international researchers visited an Early Jurassic site known for its exceptionally well-preserved fossils of marine animals. They collected dozens of specimens of fossilized ammonites, bivalves and crustaceans the size of a palm or more.
According to a study published in March 2023, after the team examined the approximately 183 million-year-old fossils using a powerful scanning electron microscope, they struggled to find pyrite in the fossils sticking out of the black shale. magazine Geoscience Reviews (will open in a new tab).
“For a long time it was believed that all [at the Posidonia Shale] was pyritized,” study co-author Rowan Martindale (will open in a new tab), assistant professor of geological sciences at the University of Texas (Utah) at Austin, told Live Science. “We selected samples that we thought would definitely be pyrite. And, lo and behold, there was some pyrite on a couple of them, but it was mostly all phosphated or yellow calcite. us who worked on the paper.”
After analyzing approximately 70 samples, it was clear that although the schists surrounding the fossils were “studded with microscopic accumulations of pyrite crystals called framboids,” it was the phosphate minerals with yellow calcite that were the source of the fossils’ golden glow. A statements (will open in a new tab).
Connected: New fossils show trilobites had a hidden third eye
“Framboids look like tiny raspberries,” study co-author. Sinjini Sinha (will open in a new tab), a doctoral student at UT’s Jackson School of Geosciences, told Live Science. “Looking at the specimens under a microscope, I found only a few framboids on the fossils themselves, but counted between 600 and 800 on the surrounding shale.”
Knowing that pyrite and phosphate are present in different parts of the samples is critical as it provides information about the fossilization environment. For example, pyrite forms in an anoxic environment where there is no oxygen, while phosphate minerals such as yellow calcite are said to require oxygen to form.
“The process of pyritization occurs only in an oxygen-free environment,” co-author of the study. James Schiffbauer (will open in a new tab), assistant professor of geology at the University of Missouri, told Live Science. “Thesis [specimens] are in dark, black hues where we would expect it to be an anoxic environment.”
The study found that while the anoxic seafloor “laid the ground for petrification”, it took an explosion of oxygen to set off the chemical reactions needed for petrification. Oxygenation, combined with phosphate minerals, helped turn fossils into what looks like gold, according to the statement.