Dendritic pyrolucite

Sometimes, when finding fossils in flaggy limestones, the bedding is subject to different forms of mineralization. This pterosaur fossil in the Natural History Museum, London, has a formation of black Manganese dioxide - MnO2 (Dendritic pyrolucite) forming crystal structures around the fossil bones. This is quite common in these Bavarian specimens, though they are not very frequently seen beyond the storage rooms of museums.

Pyrolucite - MnO2 is used in Zinc Oxide dry cell batteries as an oxidising agent. This is a stable compound formed in nature from unstable ions. It can be reduced to Manganese oxide MnO with the addition of Hydrogen.

In the Fossils, the dendrites of Pyrolucite grow away from the fossil bones, along the bedding plane of the sediment, forming a fern like pattern on the rock. The crystals are orthorhombic in form and have a hardness of 2. Excessive contact will result in staining to the fingers and the mineral has a streak colour like graphite. This crystal formation is a result of ions carried in water which is forced through the sedimentary beds during the later stages of rock formation. The fluid is forced through the rock bedding planes where it will deposit materials in cavities and structures within the rock. When the hardening sediments are under pressure stress, the areas around impurities, like fossils and hard mineral grains will form micro cavities within the nearby sediment which allows deposition of crystals. The dendritic solidification of MnO2 around the fossil is a reflection of the sedimentary stress patterns and this is seen as a series of frond like patterns emanating from the bones. Quite often, the deposition is greater on one side of the fossil, or in some cases near a small area or part of a fossil. The example shown in the photograph is an extreme case.