PYROLUSITE [ Oxides and Hydroxides : Rutile ]
- MnO2, manganese Oxide
- A major ore of manganese and as a mineral specimen
Pyrolusite is the most common manganese mineral and is an important ore. Manganese is a strategically valuable metal since it is an essential ingredient in steel and other alloys. The mining term “wad” is used to indicate ores that are a mixture of several manganese oxides such as pyrolusite, psilomelane and others that are difficult to distinguish.
Pyrolusite is an oxidation product of weathered manganese minerals and also forms from stagnant shallow marine and freshwater bog and swamp deposits. Minerals such as rhodochrosite, rhodonite and hausmannite are often replaced by pyrolusite.
Pyrolusite has some interesting habits dispite its common occurrence as dull, sooty, black masses and/or earthy forms. Possibly its most popular form is its dendritic habit that forms wonderfully detailed, fern-like patterns on the surfaces of rocks such as sandstone. These dendrites are so amazing that they have often been mistaken for fossil plants. Another popular habit is its acicular or hair-like crystal aggregates that produce nice tufts of “hair”, or meadows of shiny black pyrolusite fibers. Often specimens of pyrolusite are very difficult to distinguish from other manganese oxides. 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.
Colour: steel gray to a solid black in earthy specimens
Luster: metallic to dull in weather or thinly crusted specimens
Transparency: crystals are opaque, translucent in only thin splinters
Crystal System: tetragonal; 4/m 2/m 2/m
Crystal Habits: typically massive and compact forms, but also fibrous, acicular, columnar, concretionary, scaly and earthy forms are well known. 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: good in two directions forming prisms, but rarely seen except in rare large crystals
Fracture: conchoidal to uneven
Hardness: 6 in individual crystals, but aggregates can be as soft as 4 or 5 and massive or earthy forms will mark paper and leave powder on fingers (a hardness under 2)
Specific Gravity: 4.4 – 5.1 (average for metallic minerals)
Associated Minerals: limonite, hematite, quartz, manganite, psilomelane and other manganese and iron oxide minerals
Major Occurrences: include nice specimens from Germany; iron mines in Minnesota and Michigan and at Lake County, New Mexico, USA. Pyrolusite is mined in many countries around the world with the most productive countries being Georgia and Ukraine of the former USSR, India, China, South Africa, Brazil, Australia and Gabon. Polianite occurs in abundance at the Kisenge Mine, in Zaire
Best Indicators: habits, luster, softness, colour and streak