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Sphalerite Mineral Facts:

Chemical Formula:  ZnS  - The Mineral is 67% Zinc by weight when pure.

Colors: Usually Black, Brown or some shade of Red, may be yellowish or green.
Its streak is brownish, yellow or white. Color is white when absolutely pure, and green when nearly so, the color darkening with increase in the amount of iron present. . The yellow masses look very much like lumps of rosin.

Hardness: 3.5 to 4

Density: 4 to 4.1
Density varies somewhat depending on formation and impurities.

Cleavage: Sphalerite has a perfect dodecahedral cleavage parallel to {011}. Its fracture is conchoidal.

Crystallography: Isometric, Tetrahedral
The tetrahedron, dodecahedron and cube are all common forms, but the crystals are frequently highly complex and usually distorted or in rounded forms. They are often twinned. Commonly found in massive cleavable forms, coarse to fine granular, also compact and botryoidal. Also in rounded crystal masses.

Luster:. Adamantine to Resinous to Submetallic, transparent to nearly opaque.

 Optics: (Refractive Index)  2.369 for yellow light when transparent (a rather High RI)

Composition, Structure and Associated Minerals:
Sphalerite is the most important ore of zinc, and an extremely common mineral, especially as a constituent of metallic veins. Found widely distributed, but chiefly in veins and irregular bodies in limestone rocks. Associated with pyrite, galena, marcasite, chalcopyrite, smithsonite, calcite, dolomite, siderite, etc. It may carry silver or gold. It is one of the very important zinc ores and one of the most interesting minerals from a crystallographic standpoint, occurs in amorphous and crystalline masses and in handsome crystals and crystal groups. Botryoidal and other imitative masses are common. From a compact mass of the mineral a fairly good dodecahedron may sometimes be split. When pure the mineral is transparent and colorless. Pure white sphalerite consists of 67 per cent of Zn and 23 per cent of sulfur. However it 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. The colored varieties usually contain traces of silver, iron, cadmium, mercury, manganese and other metals. Sometimes the proportion of the impurities is so large that the mineral containing them is regarded as a distinct variety. Argentiferous zinc is the source of a considerable quantity of silver. The mineral is a nonconductor of electricity. By the effects of weathering and oxidation, sphalerite changes into the sulfate of zinc, and by other processes into the silicate of zinc, calamine, or the carbonates, smithsonite and hydrozincite.

Identification and Diagnostics
Sphalerite is difficultly fusible, and when powdered always yields tests for sulfur under proper treatment. On charcoal it volatilizes slowly, coating the coal with a yellow sublimate when hot, turning white on cooling. When moistened with a dilute solution of cobalt nitrate and heated in the reducing flame, the white coating of ZnO turns green. The mineral dissolves in hydrochloric acid, yielding sulphuretted hydrogen. Infusible with pure zinc sulfide to difficultly fusible with increase in amount of iron. Gives odor of sulfur dioxide when heated on charcoal or in 0. T. Decomposed in powder by warm hydrochloric acid with evolution of hydrogen sulfide gas, which may be detected by its disagreeable odor. When heated on charcoal gives 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.

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Occurrence, Localities and Origins:
Sphalerite occurs disseminated thorough limestone, in streaks and irregular masses in the same rock, and in veins cutting crystalline and sedimentary rocks. It is often associated with galena. The material in the veins is often crystallized. Here it is associated with chalcopyrite (CuFeS2), fluorite (CaF2), barite (BaSO4), siderite (FeCOs), and silver ores. When in veins it is in some cases the result of ascending hot waters and in other cases the product of percolating meteoric water. Much of the disseminated ore is a metamorphic contact deposit.

Crystallized sphalerite is found abundantly at Alston Moor, Cumberland, England; at various places in Saxony; in fine crystals at Binnenthal, Switzerland; at Broken Hill, N. S. Wales, and in nearly all localities for galena. It is also found at Schemnitz and other localities in the gold and silver-mining districts of Hungary. Handsome, transparent, cleavable masses are brought from Pilos de Europa, Santander, Spain. Stalactites are abundant near Galena, Illinois. The principal deposits of economic importance in America are those in Iowa, Wisconsin, Missouri and Kansas, where the sphalerite is associated with other zinc compounds and with galena forming lodes in limestone, and at the silver and gold mines of Colorado, Idaho and Montana. Large deposits are found in the United States in Missouri, Kansas, Arkansas, Wisconsin, Iowa, Illinois, Colorado. The chief US locality for its production is the Joplin District in southwestern Missouri. Found in large quantities in connection with the lead-silver deposits of Leadville, Colorado.

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