[Smithsonite]
ZnCO₃
[Pl. 21]
Dry bone
Usually occurs as incrustations, grains, earthy or compact masses, and as crystals; hardness 5; specific gravity 4.4; color white, yellow, greenish or bluish; streak white; luster vitreous; transparent on thin edges.
When pure this mineral is colorless, but, as it occurs, it is usually white, or tinged with some shade of yellow, green, or blue, but in all cases its streak is white. The crystals are rhombohedrons often with edges beveled or corners cut by other faces. It resembles calamine and willemite, but is readily separated from either of these by the acid test, for smithsonite effervesces when acid is placed on it.
Next to sphalerite, smithsonite is the commonest of the zinc minerals. It is a secondary mineral, resulting from the action of lime-charged water acting on sphalerite, and so is likely to be found wherever zinc minerals occur in a limestone region. In the Wisconsin-Illinois-Iowa district it serves as a minor ore of zinc, and is termed here “dry bone.” It is also found in the Missouri and Arkansas districts, and in Europe is an important ore for zinc.
[Franklinite]
(ZnMn)Fe₂O₄
[Pl. 21]
Occurs in compact grains or masses, and in isometric octahedrons; hardness 6; specific gravity 5; color black; streak reddish-brown; luster metallic; opaque on thin edges.
This is a mineral peculiar to the Franklin Furnace region, from which it gets its name. It looks like magnetite, but its reddish-brown streak and lack of magnetism distinguish it. When it occurs in octahedrons, the edges are rounded, while those of magnetite are sharp. It is a complex and variable oxide of zinc, iron and manganese, which has resulted from the metamorphism of the beds in which it occurred probably being originally something quite different.
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.