Occurs in needle-like or fibrous crystals; hardness 3.5; specific gravity 5.5; color brass-yellow; streak greenish black; luster metallic; opaque on thin edges.
The fibrous crystals of millerite belong to the orthorhombic system. The color and streak suggest pyrite, but the crystals are long and slender, while pyrite is in cubes, octahedrons, etc. If there is any doubt of the identity of this form, place a piece in nitric acid, and if it is millerite, it will color the acid green.
It may occur in veins associated with cobalt and silver minerals, or as a secondary mineral as at Gap Mine, Penn., or in cavities in sedimentary rocks. In the last case it usually is in needle-like crystals growing through calcite crystals, as at St. Louis, Mo., Keokuk, Iowa, and Antwerp, N. Y.
The Cobalt Group
As a metal, cobalt is hard, brittle, and of a grayish color, tinged with red. It was not recognized as a separate element until 1735, and even today is one of the minor metals. Cobalt, chromium and a little tungsten make the alloy stellite, which has come into large use in making high-speed tools. The oxide of cobalt (CoO) is “smalt,” used to give the blue color to porcelain, pottery, glass, tiles, etc. Invisible ink is made by diluting cobalt chloride in a large quantity of water. This solution is a faint pink color and practically invisible on paper, but if heated it loses water and turns blue in color, and is perfectly visible.
Cobalt is another of the metals, of which the United States does not have an adequate supply. Sweden, Norway and India were the chief sources of supply until cobalt was found near the town of Cobalt in Ontario, Canada, and now this district furnishes 90% of the world’s supply.
[Cobaltite]
CoAsS
[Pl. 26]
cobalt glance
Usually crystalline in cubes, pyritohedrons or octahedrons; hardness 5.5; specific gravity 6.1; color reddish silver-white; streak grayish-black; luster metallic; opaque on thin edges.
In color cobaltite may appear very like arsenopyrite, especially if the reddish tinge is not strong, in which case the mineral can be definitely determined by putting a piece in nitric acid. If it is cobaltite the solution will be colored rose-red, if arsenopyrite there will be no change of color. The forms of the crystals are the same as those of pyrite, but the color will easily distinguish cobaltite from pyrite. This pink color is characteristically present either in or about cobalt minerals, being sometimes called “cobalt bloom.” It is a cobalt-arsenic-oxide with water of crystallization (Co₃As₂O₈·8H₂O), which results from the exposure of cobalt and arsenic minerals to air and moisture. It is the pink color on the figures of both cobaltite and smaltite. In Sweden, Norway and India, this is the chief ore for cobalt, but in the United States it is rather rare, but is found in Oregon, and at Cobalt, Canada.