Field book of common rocks and minerals

by Frederic Brewster Loomis

The Manganese Group

Though manganese was known in the mineral pyrolusite in early times, it was then thought to be magnetite or magnetic iron ore. It was not until 1774 that it was isolated and recognized as a distinct element.

Manganese is one of the lesser elements in the crust of the earth, making less than .07 of one percent, but as an alloy with other metals, especially iron, it has attained a considerable importance to man. It is used chiefly with iron, 20% of manganese making the alloy, spiegeleisen, a combination which occurs in Nature in Germany, and from 20% to 80% making ferromanganese. These alloys are in great demand because they make an especially tough steel essential in the manufacture of munitions. The sources for manganese are the oxide ores, manganite, pyrolusite and psilomelane, which have been formed as secondary minerals, as a result of the weathering of silicates which carry manganese. They occur widely enough, but throughout the United States the deposits are small, and this is one of the elements in which this country is not self-sufficient. The largest producer of manganese is Russia; however she consumes almost all of her output at home, and our supply comes from the next largest producers, India, the Union of South Africa, and the Gold Coast. A shift in trade may be expected when Brazil’s recently discovered ore body in Matto Grosso is brought into full production. Besides being used as an alloy, manganese is employed in making paints and dyes, for clearing glass, and for some types of electric batteries.

[Pyrolusite]
MnO₂

Occurs in earthy or fibrous masses; hardness 1-2; specific gravity 4.8; color black; streak black; luster dull; opaque.

Pyrolusite occurs in soft masses and incrustations, usually leaving a sooty mark on the fingers. Sometimes it seems to be in crystals, but these are pseudomorphs which have the form of manganite, from which the pyrolusite has formed as a result of the water having been driven from the manganite. Frequently pyromorphite and manganite will be found together, and in some cases the outer part of a mass or crystal will be pyrolusite, while the center is still manganite. Psilomelane is another oxide of manganese with water and may appear very like pyrolusite, but both manganite and psilomelane have much greater hardness than does pyrolusite. If there is difficulty in deciding about pyrolusite, it may be placed in a closed tube and heated. It will not be affected by the heat, while, under the same circumstances, both manganite and psilomelane will give off water vapor.

Pyrolusite usually occurs in black streaks or pockets in residual clays which have formed as a result of the decomposition of limestones. It may also occur in dendritic forms in seams and crevices (see manganite). It is found in Vermont, Massachusetts, Virginia, Arkansas, Colorado, California, etc.

[Psilomelane]
MnO₂·H₂O

Occurs in compact botryoidal or stalactitic masses; hardness 5-6; specific gravity 4.2; color black; streak brownish-black; luster metallic; opaque on thin edges.

Psilomelane is very like pyrolusite, and often occurs with it. It is distinguished by its greater hardness, and the fact, that when heated in a closed tube, it gives off water vapor. From manganite it is more easily distinguished, for it never occurs in crystals, while the manganite is usually crystalline. This and pyrolusite are the principal ores of manganese.

Wad is an impure form of psilomelane, having some iron oxide mixed with the manganese oxide, usually limonite; or the impurity may take the form of a copper, cobalt, lithium or barium oxide.

Psilomelane is found at Brandon, Vt., in Arkansas, Colorado, California, etc.

[Manganite]
Mn₂O₃·H₂O
[Pl. 22]

Occurs in prismatic crystals, or in columnar or fibrous masses; hardness 4; specific gravity 4.4; color steel gray; streak reddish-black; luster submetallic; opaque on thin edges.

This is the form taken by manganese oxide when it crystallizes in the presence of moisture, and pyrolusite frequently changes to manganite when exposed to moisture. The crystals are orthorhombic prisms, with striated sides and the ends truncated. These prisms usually occur in bundles and give the mineral a fibrous appearance. Manganite is not hard to identify, the striations on the crystals and the streak being very characteristic.

In seams and tiny crevices this mineral, and often pyrolusite, grows in a branching manner, resembling tree-like or “mossy” masses. This is termed dendritic, and the growths of manganese minerals are called dendrites. One of the most curious of these is when the “mossy” growth is inclosed in chalcedony, making the so-called moss agate. These moss agates are abundant through the Rocky Mountains and are frequently cut for semi-precious stones. The finest ones however come from India and China.

Manganite is found in the Lake Superior region, Colorado, etc.

[Rhodochrosite]
MnCO₃

Occurs in compact cleavable masses; hardness 4; specific gravity 3.5; color rose to dark red; streak white; luster vitreous; translucent on thin edges.

This usually occurs in pink to red masses which cleave readily parallel to the faces of the rhombohedron. When it is found in crystals, which are rare, these too are rhombohedrons. It is usually found in veins as a gangue mineral with copper, silver or zinc ores. Its beautiful color and the fact that it effervesces in acid serve to distinguish this mineral. It is found at Branchville, Conn., at Franklin Furnace, N. J., and in veins with silver in Colorado, Nevada, and Montana.