Fossils on roofing slates

Hello slate people

Fossils are the remains of organisms that lived many million years ago, witnesses of a lost world. Due to its particular formation conditions, slate is a perfect record to preserve fossils. Slate is formed from fine grained marine sediments, deposited in deep and calm waters, with no light or oxygen. Occasionally, dead organisms were buried in these sediments. Because of the absence of oxygen, these bodies did not rotten, and were preserved as fossils. Sometimes, the quality and detail of these fossils is so good that the outcrop becomes more known by them than by the slate itself. In the outcrops of Hünsruck, Germany, geologists found an astonishing accumulation of well-preserved fossils, of several different species. These quarries had been worked for roofing slates since at least the 14th century, and findings of “strange animals” were common for the miners, but until the second half of the 19th century the origin and meaning of these animals was unknown. Hünsruck is famous for its starfishes, crinoids and arthropods. Another important slate outcrop with exceptional fossils is Arouca, in Portugal, source of some of the biggest trilobites in the world. There is a small museum some hundred meters far from the quarry, where you can admire the fossils, a perfect family visit.

Fossils are very common on regular roofing slates, especially on black slates, but sometimes difficult to recognize. Usually, these fossils, mainly small shells, are transformed into pyrite or pyrrhotite, due to a geological process of mineral replacement known as pyritization. Carbonates and phosphates, minerals that form the hard parts of the organisms, are replaced by iron sulphides. The result is that most of pyrites you can see on a roofing slate are that, small fragments of shells, impossible to identify as fossils for anyone but a palaeontologist (and perhaps a geologist). In roofing slates, we mainly have small cubes of golden pyrite, which is very unlikely to oxidize, while pyrrhotite can be recognized as amorphous, brownish irregular grains, sometimes very difficult to spot. This is known among the sector as “rusty pyrite” or “unstable pyrite”.

So pay attention next time you see a pyrite/pyrrhotite cluster on the surface of a slate, it is probably a fossil. Take a detailed look and try to identify the remains of a living animal, sometimes is possible.

Have fun!

Image Fossils_01: Shell of an Orthoceras in a Spanish slate
Image Fossils_02: Small fragments of shells replaced by pyrrhotite

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