The Copper Group
After gold the next metal to be utilized was copper. About 4000 B.C. our early forefathers found that by heating certain rocks, they obtained a metal which could be pounded, ground and carved into useful shapes. Curiously enough the rocks which had the copper also had some tin in them, so that this first-found copper was not pure, but had from five to ten per cent of tin in it, making the resulting metal harder, and what we call bronze. It was some thousands of years later before they distinguished the copper as a pure metal, but it worked and made good tools. The newly found metal was not as ornamental as gold; but, because it could be made into tools, it had a tremendous influence on man’s development. As the bronze tools began to take the place of the stone implements, the “Age of Bronze” was ushered in. In America the Indians in the Lake Superior region found native copper weathered out of the rocks and later mined it, and they too pounded it into knives, axes, needles, and ornaments, but probably never learned to melt it and mold their tools. At any rate they were not as far advanced in using this metal when Columbus landed as were the southern Europeans 6500 years earlier. Since the use of iron became general, copper has not held such a dominant place, but it still is “the red metal” which holds the second most important place.
It is malleable and ductile, though not equal to gold or silver in these respects. It is a good conductor of electricity and a very large amount of copper is used in electrical manufacture, roofing, wire, etc. It alloys with other metals; ten parts copper and one of tin being bronze, ten of copper and one of zinc is brass, and copper with aluminum is aluminum bronze.
Like silver and gold, copper is widely diffused through the igneous rocks, but before it is available, it must be leached out by solvents and concentrated in veins, fissures, or definite parts of the lavas or granites. The primary ores are those which, while the igneous rock was still hot, were carried by hot vapors and liquids into the fissures and there deposited, mostly as sulphides. There is a long list of these, but in this country, the following are the commoner ones; chalcocite the sulphide of copper, chalcopyrite the sulphide of copper and iron, bornite another combination of copper, iron and sulphur, and tetrahedrite copper and antimony sulphide. When these primary ores are near enough to the surface to come in contact with waters carrying oxygen, carbon dioxide or silica in solution, they may give up their sulphur and take some one of these new elements and we have such forms as cuprite, the oxide of copper, malachite and azurite, carbonates of copper, or chrysocolla, the silicate of copper. Native copper is also a secondary deposit laid down in its present state by a combination of circumstances which deprived it of its original sulphur. In general copper mining can not be profitably carried on for ores with anything less than a half of one percent in them; and the use of such low grade ores has only been possible for a few years, as the result of inventing most delicate processes in the smelting.
The United States produces about a quarter of the world’s supply of copper, with Chile ranking second with about 17%.
[Copper]
Cu
[Pl. 8]
Usually in irregular masses; hardness 2.5; specific gravity 8.9; color copper red; luster metallic; opaque. Native copper, easily determined by its color and hardness, is generally found in irregular grains, sheets, or masses, on which may sometimes be detected traces of a cube or an octahedral face, showing that it belongs to the isometric system. The most famous locality is the Upper Peninsula of Michigan which may be taken as typical. Here, long before it was known historically, the Indians found and dug out copper to make knives, awls, and ornaments.
In this region, beds of lava alternate with sandstones and conglomerates. The copper was originally in the lavas, but has been dissolved out, and now fills cracks and gas cavities in the lavas, and also the spaces between the pebbles of the conglomerate. This locality has been very famous both because of the quantity mined, and also because of the strikingly large masses sometimes found. Today but little of the ore runs above 2 percent copper, and it is mined if it has as little as ½ of one percent.
While nowhere near as abundant, native copper occurs in the same way in cavities and cracks in the trap rocks of New Jersey, and along the south shore of the Bay of Fundy. It is also known from Oregon, the White River region of Alaska, and in Arctic Canada.
[Chalcopyrite]
CuFeS₂
[Pl. 8]
copper pyrites or yellow copper ore
Occurs in crystals of irregular masses; hardness 4; specific gravity 4.2; color bronze yellow; streak greenish black; luster metallic; opaque on thin edges.
Chalcopyrite resembles pyrite, but its color is a more golden yellow, and its surface tarnishes with iridescent colors. Then too the hardness of chalcopyrite is but 4 as compared with 6 for pyrite. When in crystals this mineral belongs to the tetrahedral system as the c axis is but .985 in length as compared with I for the two other axes. This difference is so little that, to the eye, the octahedron appears to belong to the isometric system. Chalcopyrite occurs in octahedrons and tetrahedrons (as on [plate 8]), the latter being the form where but half of the octahedral faces are developed. However by far the most frequent mode of occurrence is in irregular masses.
This is the most important primary ore of copper, and is widely distributed, being found either in lavas, or in veins, or in fissures connected with igneous rocks. Apparently the deposits were made, either at the time of eruptive disturbances or shortly afterward, from vapors or hot solutions carrying the copper sulphides (and other sulphides) from the molten igneous rocks. Chalcopyrite is usually associated with pyrite, galena, sphalerite and chalcocite, as well as quartz, fluorite and calcite. It is found in all the New England States, in New York, New Jersey, Pennsylvania, Maryland, Virginia, North Carolina, Tennessee, Missouri, and all the Rocky Mountain and Pacific Coast States.
[Bornite]
Cu₃FeS₃
purple copper ore
Occurs in granular or compact masses; hardness 3; specific gravity, 5; color bronze-brown with a bluish tarnish; streak gray-black; luster metallic; opaque on thin edges.
Bornite is also known as erubescite, blushing ore, variegated copper, peacock copper, etc., all of which names refer to the highly iridescent tarnish which fresh faces soon take on when exposed to the air. Though usually in masses, it is sometimes found in rough cubes of the isometric system. In this country it is not abundant enough to be used as an ore, but is likely to be found with other ores like chalcopyrite or chalcocite. In the east it has been found at Bristol, Conn., and near Wilkesbarre, Penn., while in the west it may be expected to occur wherever other sulphide minerals of copper are found.
[Chalcocite]
Cu₂S
[Pl. 9]
copper glance
Occurs in fine grained compact masses; hardness 2.5; specific gravity 5.7; color dark leaden gray; streak black; luster metallic; opaque on thin edges.
Chalcocite is one of the important ores of copper, especially in Arizona and the Butte District of Montana. It resembles argentite in color and general appearance, but is readily distinguished by being brittle and having a tendency to tarnish to bluish or greenish colors on fresh surfaces. Occasionally it occurs in crystals which are in the orthorhombic system; but the edges of the prism are so beveled that there are six sides and the prism resembles a hexagonal prism (see [page 16]).
In the Butte, Mont., district, the most important copper region in the United States, fully 50% of the ore is chalcocite, which is a derivative of the originally deposited chalcopyrite, the latter having lost its iron. In the veins of this district chalcopyrite, bournite, tetrahedrite, and several other copper minerals not described in this book, occur all together, and with them also gold, silver and arsenic minerals. The gold amounts to about 2¼ cents per pound of copper, and the silver is in somewhat less quantity. These veins were first opened to get the silver ores, which were the more important ones down to a depth of 200 to 400 feet. Below these depths the copper became much more important. It was the weathering which had removed a large part of the copper minerals in the upper levels of the veins, but had left a large part of the silver. Chalcocite is also important in most of the Utah and Arizona mines.
In the east it has been found at Bristol, Simsbury and Cheshire, Conn., and in the west it is found in all the Cordilleran States.
[Tetrahedrite]
Cu₃SbS₃
Pl. [9] & [10]
gray copper ore
Occurs in irregular masses and in tetrahedrons of the isometric system; hardness 3.5; specific gravity 4.7; streak dark brown; luster metallic; opaque on thin edges.
In its crystalline form the tetrahedrite occurs in tetrahedrons, which generally have faces formed by beveling the edges and by cutting the corners, as in the two figures of [plate 10]. Chalcopyrite may also occur in tetrahedrons, but its golden yellow color is entirely different from the gray-black of the tetrahedrite. When in masses the hardness and the streak which is dark brown, are very characteristic.
In England and Bolivia tetrahedrite is an important ore of copper, but in this country it is simply a copper mineral which is widely distributed, and associated with most of the mining enterprises, but is in no case the important ore. It has been found sparingly through the New England States, at the Kellogg Mines in Arkansas, and abundantly in Colorado, Montana, Utah, Arizona, Nevada and New Mexico.
[Cuprite]
Cu₂O
Pl. [9] & [10]
red copper ore
Occurs in isometric cubes, octahedrons, and dodecahedrons, or in masses; hardness 3.5; specific gravity 6; color dark brownish-red; streak brownish-red; luster metallic; translucent on thin edges.
When in crystals cuprite is easily determined, but when in masses its fresh surfaces may suggest prousite, but the streak and hardness are quite different in the two cases. Sometimes its color suggests hematite, but the latter has the hardness of 6. When found it is often coated with a thin film of green, which is malachite.
Except when found as native copper, the ore which contains the greatest percentage of copper is cuprite with 88.8% of copper. It is likely to occur in any of the deposits of copper ore, where they are in arid climates and above the level of the underground water, and is very frequently associated with malachite and azurite. In the Bisbee, Arizona, district cuprite is one of the important ores.
Besides the normal occurrence described above, cuprite may be found in two other varieties; one where the crystals have grown side by side and so only the ends have been free for continuous additions of the mineral, which has resulted in a fibrous mass known as “plush copper ore” or chalcotrichite; the other an earthy mixture of limonite and cuprite, which is brick red in color, and termed “tile ore.”
Cuprite is found sparingly in New England, more abundantly at such places as Summerville and Flemington, N. J., Cornwall, Penn., in the Lake Superior region, and fairly abundantly in the Cordilleran States.
[Malachite]
CuCO₃·Cu(OH)₂
[Pl. 11]
Usually occurs in nodular or incrusting masses; hardness 3.5; specific gravity 4; color green; streak a lighter green; luster adamantine, silky or dull; translucent on thin edges.
The vivid green of malachite is usually enough to determine it at once, but one may be sure by trying a drop of acid on it, in which case it effervesces as is characteristic of so many carbonates, but this is the only carbonate which is vivid green. Generally the malachite is in irregular masses, but crystals are occasionally found. These are extremely small and needle-like, and belong to the monoclinic system. In the Ural Mountains there is a locality where these crystals grow in fibrous masses, usually radiating from the center. Malachite in such nodules has a silky luster. These rare nodules have furnished the rulers of Russia with a unique and much prized material for making royal gifts. In European museums and palaces one finds many objects carved from this form of malachite, and marked as gifts of the czars of Russia.
In the United States malachite is widely distributed, appearing as green streaks and stains where copper minerals have been exposed to the air. It is the green tarnish which appears on bronze and copper when exposed to the weather. It is found in large quantities in New Jersey, Pennsylvania, Wisconsin, Nevada, Arizona, Utah, New Mexico, etc. The Bisbee mine in Arizona is the place that has furnished museums with so many of the handsome specimens of malachite associated with azurite. These are the most striking specimens for the vividness of their colors that appear in any collections.
Malachite has been known since about 4000 B.C., the Egyptians having mines where they obtained it between the Suez and Mt. Sinai. In those early days it was particularly a child’s charm, protecting the wearer from evil spirits. It is still used as a stone of lesser value in making some sorts of jewelry.
[Azurite]
2CuCO₃·Cu(OH)₂
[Pl. 11]
Occurs as short prismatic or tabular crystals of the monoclinic system; hardness 4; specific gravity 3.8; color azure blue; streak lighter blue; luster vitreous; translucent on thin edges.
Azurite is another very striking mineral fully characterized by its color and streak. Like malachite it effervesces in acid. It is very near to malachite in composition, and by increasing its water content, can and freely does change to the green mineral; so that few specimens of azurite are without traces of malachite. It is found in the same places as malachite, but is not as abundant in the east.
Azurite with the accompanying malachite is cut and polished to make semi-precious stones for some forms of jewelry.
[Chrysocolla]
CuSiO₃·2H₂O
Never occurs in crystals, but in seams and incrustations; hardness 2-4; specific gravity 2.1; color bluish-green; streak white; luster vitreous; translucent on thin edges.
This rather rare mineral often appears in opal- or enamel-like incrustations, and its color is variable ranging from the typical bluish-green to sky-blue or even turquoise blue. This is a mineral resulting from the action of silica bearing waters, coming in contact with most any of the copper minerals, and is found accompanying cuprite, malachite, azurite, etc. It is never in large enough quantities to be used as an ore, but its striking color attracts attention and it can be found fairly frequently, especially in the west.