Sphalerite Group. Isometric, Tetrahedral, Zinc Blende, Black Jack

Sphalerite. Zinc Blende, Black Jack.

Composition. Zinc sulphide, ZnS = Sulfur 33, zinc 67., Almost always contains at least a small percentage of iron replacing the zinc, but the amount of iron may rise as high as 15 to 18 per cent. Also frequently contains small amounts of manganese, cadmium, mercury, etc. (see p. 84).

Crystallization. Isometric; tetrahedral. Tetrahedron (Fig. 225), dodecahedron and cube common forms, but the crystals frequently highly complex and usually distorted or in rounded forms. Often twinned.

Structure. Usually massive cleavable, coarse to fine granular. Compact, botryoidal. Also in rounded crystals masses.

Physical Properties. Perfect dodecahedral cleavage. H. = 3.5-4. G. = 4-4.1. Nonmetallic and resinous to submetallic luster; also adamantine. Color white when pure, and green when nearly so. Commonly yellow, brown to black, darkening with increase in the amount of iron present. Transparent to translucent. Streak white to yellow and brown.

Test. Infusible with pure zinc sulphide to difficultly fusible with increase in amount of iron. Gives odor of sulfur dioxide when heated on charcoal or in O.T. Decomposed in powder by warm hydrochloric acid with evolution of hydrogen sulphide gas, which may be detected by its disagreeable odor. When heated on charcoal fives a coating of zinc oxide (yellow when hot, white when cold) which is nonvolatile in oxidizing flame. Recognized usually by its striking resinous luster and perfect cleavage. The dark varieties (black jack) can be told by noting that a knife scratch leaves a reddish brown streak.

Occurrence. Sphalerite, the most important ore of zinc, is an extremely common mineral, especially as a constituent of metallic veins. In its occurrence and mode of origin it is closely allied with galena with which it is most commonly associated. Found widely distributed, but chiefly un veins and irregular masses in limestone rocks where the ore bodies have been formed largely by replacement processes. It is also found in veins in eruptive rocks; in contact metamorphic deposits; etc. Associated with galena, pyrite, marcasite, chalcopyrite, smithsonite, calcite, dolomite, siderite, etc.

May carry silver or gold. Large deposits are found in the United States in Missouri, Colorado, Montana, Wisconsin, Idaho and Kansas. The chief locality for its production is the Joplin District in southwestern Missouri and in adjacent districts in Kansas and Oklahoma. Noteworthy European localities are at Alston Moor and other places in the lead-mining districts of northern England; Binnesthal, Switzerland, in fine crystals; at Schemnitz and other localities in the gold and silver-mining districts of Czechoslovakia and Rumania.

Name. The name blende is form the German, blind or deceptive, because while often resembling galena it yielded no lead. Sphalerite, for the same reason, is derived from a Greek word meaning treacherous.

Use. the most important ore of zinc. The chief uses for metallic zinc, or spelter, are in galvanizing iron, making brass, an alloy of copper and zinc, in electric batteries, and as sheet zinc. Zinc oxide, or zinc white, is used extensively for making paint. Zinc chloride is used as a preservatite for wood. Zinc sulphate is used in dyeing and in medicine. Sphalerite also serves as the most important source of cadmium.