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.
This mineral is one of those resulting from metamorphic alteration and is derived from calamine, when the latter loses its water of crystallization. It is common at Franklin Furnace, N. J., and also found occasionally elsewhere, as at Salida, Colo., and in Socorro Co., New Mexico.
[Calamine]
Zn₂(OH)₂·SiO₃
Occurs as crystalline linings in cavities, or as botryoidal or stalactitic masses; hardness 5; specific gravity 3.4; colorless to white; luster vitreous.
Calamine resembles both smithsonite and willemite when in non-crystalline masses. From the smithsonite it is easily separated by the fact that in nitric acid the smithsonite effervesces and the calamine does not. From willemite it is harder to distinguish, but a piece may be placed in a closed tube and heated. If it is calamine water vapor will be given off, if willemite nothing happens. When calamine occurs in crystals these are orthorhombic and mostly tabular, and the crystals are peculiar in that the two ends are terminated differently.
Both this and smithsonite are secondary minerals and usually occur together when zinc is found in limestones. It is abundant at Franklin Furnace and Sterling Hill, N. J., and also found at Phœnixville, Penn., in Wythe Co., Va., and Granby, Mo.