Pyrolusite
Image: “Jonathan Zander (<a href="//commons.wikimedia.org/wiki/User:Digon3" title="User:Digon3">Digon3</a>)" (CC BY-SA 3.0)
Description
This specimen features black, fern-like pyrolusite dendrites intricately patterned across a light greenish-gray to brownish matrix, showcasing the mineral's characteristic arborescent growth habit. The dendrites are well-defined and spread across the surface of the approximately 3 ½ inch (9 cm) tall rock.
Geological Context
Pyrolusite is a common secondary manganese mineral, typically forming in oxidizing environments near the Earth's surface, often as a weathering product of other manganese minerals. Dendritic formations like these commonly occur in fractures and bedding planes of sedimentary rocks, where manganese-rich solutions precipitate in a tree-like pattern.
