The Zinc Group
Zinc and copper made the brass of early Roman times; but even then, zinc was not known as a separate metal, the brass being made by smelting rocks in which both zinc and copper occurred, the zinc never being isolated until much later. Some time in the later Roman times it seems to have been obtained separately, but then and all down through the Middle Ages zinc and bismuth were confused. Our earliest record of zinc being smelted, as we know it today, was about 1730 in England. In those earlier days, the product, zinc, or bismuth, or both together, were known as “spelter,” and this name has clung to zinc in mining and commercial circles; so that today, if one looks for quotations in the newspaper, he often finds zinc under the head of spelter.
Zinc, like lead, is diffused in small quantities through all the igneous rocks. In places it is segregated in fissures or veins leading from the igneous rocks, along the contact between igneous rocks and either sedimentary or metamorphic rocks, in limestones where solution cavities have been formed and later filled with zinc minerals, and as a residue where limestones have been weathered away. In all these places it is closely associated with lead.
The sulphide, sphalerite, is the primary mineral, and the other minerals, like zincite, smithsonite, calamine, willemite, franklinite, etc., are secondary, resulting from modifications of the original sphalerite. In connection with zinc minerals the region of Franklin Furnace, N. J., is especially interesting, for at that place are found two large metamorphosed deposits containing a wide range of zinc minerals, several of which are not found anywhere else.
Zinc is soft and malleable, but is only slightly ductile, and has little tensile strength. It alloys with several metals, and in this form is most useful today; three parts of copper to one of zinc making brass; four or more parts of copper and one of zinc, making “gold foil”; copper and zinc (a little more zinc than copper) making “white metal”; three parts of copper to one of zinc and one of nickel making German silver; etc. Zinc is also used in large quantities in galvanizing iron, sheets of iron being dipped into melted zinc and thus thinly coated. It is also used in batteries and a wide range of chemical industries.
[Sphalerite]
ZnS
Pl. [19] & [20]
zinc blende, black jack
Occurs in grains, in fibrous or layered masses, or in isometric crystals; hardness 3.5; specific gravity 4; color yellow-brown to almost black; streak light yellow to brownish; luster resinous to adamantine; translucent on thin edges.
When in crystals sphalerite occurs most commonly either in dodecahedrons or in tetrahedrons (hemihedral forms of the isometric octahedron). The cleavage is fairly good and parallel to the faces of the dodecahedron. The difficulty usually is to get large enough crystalline masses to see this cleavage clearly, but by examining the angles between the faces of cleavage pieces they will be found to be the same as those on a dodecahedron. When the mineral is pure, it has the color of resin, but sometimes it is reddish to red-brown, and then it is called “ruby zinc,” more often it is dark brown due to the presence of iron as an impurity. This is what the miners call “black-jack.” The presence of iron also tends to make the streak darker. The hardness, streak and cleavage will usually determine this mineral readily.
Sphalerite is the primary ore of zinc and is usually found in fissures and veins leading from masses of igneous rocks, or along the surface of contact where igneous rocks like granite or lavas come against such metamorphic rocks as gneisses, schists, or crystalline limestones. In the region of Joplin, Mo., however, the sphalerite is of secondary character, having been gathered by waters circulating through the limestones, and deposited in them in irregular pockets. This Joplin district has produced more zinc than any other in the world. The United States annually produces about 25% of the world’s supply of this metal.
Sphalerite is always associated with galena, and such other minerals as argentite, pyrite, chalcopyrite, fluorite, quartz, calcite and barite, are very apt to be present. It will be found in almost every state, especially in fissures and veins, and less frequently in cavities in limestones.
[Zincite]
ZnO
Pl. [19] & [20]
red zinc ore
Usually occurs massive, but may be found in crystals; hardness 4; specific gravity 5.6; color deep red; streak orange; luster subadamantine; translucent on thin edges.
When in crystals zincite forms in hexagonal prisms with hexagonal pyramids on the ends. This is rather rare, most of the zincite being found in massive form. The cleavage is parallel to the prism faces and perfect. The deep red color and orange streak are wholly characteristic.
This mineral is so common at Franklin Furnace, N. J., as to be an important ore, but it is very seldom found elsewhere. This district, as mentioned before, is a peculiar one for zinc minerals. The zinc beds are in a metamorphosed limestone, and into this are intruded numerous dikes of granite. Probably the zinc was originally present in the bed of limestone as smithsonite, calamine and other secondary minerals of zinc. When intruded by the hot granite the smithsonite (carbonate) may well have been altered to the oxide, zincite; while the calamine (hydrous silicate) became the simple silicate, willemite.
[Willemite]
ZnSiO₄
[Pl. 20]
Occurs in masses or in crystals; hardness 5.5; specific gravity 4.1; color pale yellow when pure; luster resinous; translucent on thin edges.
Willemite is another of the minerals which are distinctively characteristic of Franklin Furnace, and found elsewhere very rarely. It is so common there as to be one of the principal ores, and mostly occurs in irregular masses, but is also found in crystals. These are hexagonal prisms, with a three-sided (rhombohedral) pyramid on the ends. The color when pure is whitish or greenish-yellow, but with small amounts of impurities it may be flesh-red, grayish-white or yellowish-brown. When in crystals it is easily determined; but when massive it looks like calamine, and can only be distinguished by placing a bit of the mineral in a closed tube and heating it, in which case calamine will give off water vapor, while willemite will not.
This mineral is one of those resulting from metamorphic alteration and is derived from calamine, when the latter loses its water of crystallization. It is common at Franklin Furnace, N. J., and also found occasionally elsewhere, as at Salida, Colo., and in Socorro Co., New Mexico.
[Calamine]
Zn₂(OH)₂·SiO₃
Occurs as crystalline linings in cavities, or as botryoidal or stalactitic masses; hardness 5; specific gravity 3.4; colorless to white; luster vitreous.
Calamine resembles both smithsonite and willemite when in non-crystalline masses. From the smithsonite it is easily separated by the fact that in nitric acid the smithsonite effervesces and the calamine does not. From willemite it is harder to distinguish, but a piece may be placed in a closed tube and heated. If it is calamine water vapor will be given off, if willemite nothing happens. When calamine occurs in crystals these are orthorhombic and mostly tabular, and the crystals are peculiar in that the two ends are terminated differently.
Both this and smithsonite are secondary minerals and usually occur together when zinc is found in limestones. It is abundant at Franklin Furnace and Sterling Hill, N. J., and also found at Phœnixville, Penn., in Wythe Co., Va., and Granby, Mo.
[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.