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.