pyrolusite

[German Pyrolusit : Greek puro-, pyro- + Greek lousis, a washing (from louein, to wash).]

A mineral having composition MnO₂. Well-developed crystals (polianite) arerare; it is usually in radiating fibers or reniform coatings. The hardness is 1–2 on the Mohs scale (often soiling the fingers) and the specific gravity is 4.75. The luster is metallicand the color iron-black. It frequently forms pseudomorphs after other manganese minerals, notably manganite.

Pyrolusite is extensively mined as a manganese ore in many countries, chiefly in Russia, Ghana, India, the Republic of South Africa, Morocco, Brazil, and Cuba. See also Manganese; Manganite.

Environment

Secondary manganese deposits and secondary veins.

Rarely in prismatic or stubby well-formed crystals. Sometimes in fibrous crystals and usually in fibrous masses that are pseudomorphous after other manganese oxides. Also massive, fibrous; and as black powdery to granular masses.

Steel gray to iron black. Luster metallic; hardness 6-6Ɖ (for crystals) to as little as 2 (for massive material), specific gravity 4.4-5.0; streak black (soft material blackens the fingers); fracture uneven; cleavage prismatic. Brittle.

Manganese dioxide (63.2% Mn, 36.8% O), often with a small amount of water, heavy metals, phosphorus, and other elements.

Infusible on charcoal; dissolves in hydrochloric acid with the evolution of acrid chlorine gas. Borax bead test is easy, showing in the oxidizing flame a fine amethystine color. (Avoid getting too much and having a black bead.)

The sooty black character of the streak and the manganese tests prove presence of manganese, but it is virtually impossible to tell one manganese oxide mineral from another without distinct crystals, except by x-ray tests. Pyrolusite is a safe name for any fibrous-looking mass of black manganese oxide needles or for the black powdery alterations of other manganese minerals.

Pyrolusite is the most common and most important secondary ore of manganese. It forms under conditions of oxidation, either from primary manganese minerals such as the carbonate rhodochrosite, the silicate rhodonite, and the numerous manganese phosphates or as direct deposits from cold ground water in bogs and on the sea floor. It is usually the mineral responsible for fernlike markings commonly observed along rock fissures. These are known as dendrites and are often mistaken for fossil ferns. The mineral is extremely widespread; good specimens are found in some of the Minnesota and Michigan iron ores.

Pyrolusite
Pyrolusite Mineral with Dendrite (height of sample ~9 cm)
General
Category	Oxide mineral
Chemical formula	manganese dioxide, MnO2
Identification
Color	Darkish, black to lighter grey, somethimes bluish
Crystal habit	Granular to massive: botryoidal and dendritic. Crystals rare
Crystal system	Tetragonal
Twinning	{031}, {032} may be polysynthetic
Cleavage	Perfect on 110
Fracture	Brittle
Mohs scale hardness	6–6.5, 2 when massive
Luster	Metallic, dull to earthy
Streak	Black to bluish-black
Specific gravity	4.4–5.06
Refractive index	Opaque
References	[1][2][3]
Major varieties
Polianite	pseudomorphic after manganite[4]

Pyrolusite is a mineral consisting essentially of manganese dioxide (MnO₂) and is important as an ore of manganese. It is a black, amorphous appearing mineral, often with a granular, fibrous or columnar structure, sometimes forming reniform crusts. It has a metallic luster, a black or bluish-black streak, and readily soils the fingers. The specific gravity is about 4.8. Its name is from the Greek for fire and to wash, in reference to its used to remove tints from glass.^[3]

Contents 1 Occurrence 2 Use 3 Variations in crystal habit 4 References

Occurrence

Pyrolusite and romanechite are among the most common manganese minerals. Pyrolusite occurs associated with manganite, hollandite, hausmannite, braunite, chalcophanite, goethite and hematite under oxidizing conditions in hydrothermal deposits. It also occurs in bogs and often results from alteration of manganite.^[3]

Use

The metal is obtained by reduction of the oxide with sodium, magnesium, aluminium, or by electrolysis. Pyrolusite is extensively used for the manufacture of spiegeleisen and ferromanganese and of various alloys such as manganese-bronze. As an oxidizing agent it is used in the preparation of chlorine and disinfectants (permanganates) and for decolorizing glass. When mixed with molten glass it oxidizes the ferrous iron to ferric iron, and so discharges the green and brown tints. As a coloring material, it is used in calico printing and dyeing; for imparting violet, amber, and black colors to glass, pottery, and bricks; and in the manufacture of green and violet paints.

Variations in crystal habit

Botryoidal	Dendritic
Acicular radiating	crystal structure of pyrolusite

References

1. ^ http://webmineral.com/data/Pyrolusite.shtml Webmineral data
2. ^ http://www.mindat.org/min-3318.html Mindat
3. ^ ^{a b c} http://rruff.geo.arizona.edu/doclib/hom/pyrolusite.pdf Handbook of Mineralogy
4. ^ http://www.mindat.org/min-6101.html Mindat polianite

This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.

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