Pyrolusite is often soft and soils the fingers, but when well crystallized it is harder than glass. Pyrolusite is associated and intergrown with iron oxides such as hematite and goethite and other manganese oxides such as romanechite. It may be part of the mixture of manganese minerals generally referred to as psilomelane.
It has formed in these mine workings as a modern precipitate from saline mine waters coating not only the walls and floors of the mines, but also broken rocks, nails and steel railings. Dickey, Heyl, et al. Thus, as a consequence of its more abundant distribution, pyrolusite is the default name for black, hair-like manganese crystals or powdery black alteration products of manganese minerals in general.
Luster is metallic to dull in weather or thinly crusted specimens. Transparency crystals are opaque, translucent in only thin splinters. A thin dendritic habit is commonly seen encrusted on sandstones and siltstones and will form wonderful fern or tree like patterns that are often mistaken for fossil plants. A variety with large, easily visible crystals is called polianite , and occurs as minute prismatic crystals with a square or rectangular cross-section and a wedge-shaped terminations.
Cleavage is good in two directions forming prisms, but rarely seen except in rare large crystals. Nant Uchaf Mine. Photo D. Introduction: a manganese oxide belonging to the rutile group of minerals, it is trimorphous has the same composition but a different atomic strucure with akhtenskite and ramsdellite. Occurrence in Wales: pyrolusite is probably widespread in Wales, although there are few specific records of its occurrence.
Beddgelert, Gwynedd: in a similar setting to Mynydd Nodol, blocky crystals of pyrolusite are developed in cavities within a massive to botryoidal cryptomelane group phase T. NMW King Collection.
Snowbrook Mine, Llanidloes, Powys: a pebble containing black manganese oxides dominated by pyrolusite was found near to Snowbrook Mine, but the rounded nature of this specimen suggests that it has been transported some distance and is not from the mine itself T.
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