Pyrolusite

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Formula: Mn⁴⁺O₂
Simple oxide, rutile group, manganese-bearing mineral
Polianite is a name for pyrolusite pseudomorphs after pseudoorthorombic manganite crystals (actually manganite has monoclinic structure) (Mindat).
Crystal System: Tetragonal
Specific gravity: 5.04 to 5.08 measured, 5.189 calculated
Hardness: 6
Streak: Black
Colour: Silver-grey to flat black
Solubility: Moderately soluble in hydrochloric acid; insoluble in nitric acid
Environments:

Sedimentary environments (commonest)
Hydrothermal environments

Pyrolusite is one of the commonest manganese minerals, formed under oxidising conditions and high pH (highly alkaline), (Dana) as an alteration product of manganese minerals such as rhodochrosite, and also as a chemical precipitate in lakes. Nodular deposits of pyrolusite are found in bogs, on lake bottoms, and on the floors of seas and oceans. Beds of manganese ores are found enclosed in residual clay, derived from the decay of manganiferous limestone. Pyrolusite is also found in the oxidation zone of epithermal (low temperature) hydrothermal veins with quartz and various metallic minerals.

At Burdell Gill, Coombe Height, Caldbeck, Allerdale, Cumbria, England, UK, pyrolusite is a supergene mineral associated with romanèchite in a goethite, cryptomelane and pyrolusite impregnated quartz vein. It may contain quartz inclusions, and is often covered by crusts of cryptomelane. There are possibly pseudomorphs of pyrolusite after manganite. (JRS 8(1).1-9).

At the Clargillhead vein, Garrigill, Alston Moor, Eden, Cumbria, England, UK, pyrolusite occurs as rare fibrous aggregates up to 60 microns in diameter in the quartz-fluorite gangue (JRS 23.51).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, pyrolusite is associated with palygorskite and dolomite (JRS 20.24).

At the Peldar Tor quarry, Spring Hill, Whitwick, Leicestershire, England, UK, pyrolusite occurs on quartz (RES p201).

In the Mendip Hills, Somerset, England, UK, pyrolusite occurs in mineralised cavities in manganese pods and veins. (JRS 13.15).

In the geodes at Las Choyas, Chihuahua, Mexico, pyrolusite formed on the surface of a chalcedony variety agate band, and was then covered by more chalcedony. (R&M 85.2.112).

In the Tombstone Mining District, Arizona, USA pyrolusite occurs as inclusions in calcite, and associated with quartz, bromian chlorargyrite, chlorargyrite and cerussite. Where cerussite and pyromorphite dominate, there is a strong goethite-muscovite variety illite-pyrolusite alteration. (R&M 90.4.345).

At the Manganese Mine, Keweenaw County, Michigan, USA, pyrolusite occurs in calcite vugs pyrolusite replaces braunite and macfallite. The paragenetic sequence is calcite, orientite, macfallite, braunite and manganite. Pyrolusite is a late-stage mineral, but there is much overlapping. (R&M 89.6.510).

At the Apex Mine, Lander County, Nevada, USA, pyrolusite forms a substrate for autunite on monzonite (R&M 87.3.270-276).

Alteration

pyrolusite and quartz to braunite and O₂
7Mn⁴⁺O₂ + SiO₂ ⇌ Mn²⁺Mn³⁺₆O₈(SiO₄) + 2O₂
Increasing temperature favours the forward reaction (AM80.571).

rhodochrosite and O₂ to pyrolusite and CO₂
2MnCO₃ + O₂ ⇌ 2Mn⁴⁺O₂ + 2CO₂
Increasing temperature favours the reverse reaction (AM80.571).

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