Identifying Zinc Minerals

1. Oxide Coating on Charcoal. Metallic zinc is easily obtained from the zinc minerals by fusing them with sodium carbonate on charcoal in the reducing flame. But, since the metal is volatilized at a temperature considerably bellow that of the blowpipe flame, no metallic globule can be formed. The metallic zinc is therefore all volatilized, and meeting the oxygen of the surrounding air, is converted into the oxide, ZnO, which drops upon the charcoal as a nonvolatile coating, which is yellow when hot but white when cold. The coating deposits very close to the fusion. It may frequently be obtained in more distinct form by making the fusion on a loop of platinum wire, which is held about one-quarter of an inch from the surface of a charcoal block and the blowpipe flame so directed that the oxide coating is deposited upon the charcoal behind the bead. If the coating is moistened with a drop of cobalt nitrate and then heated intensely by the blowpipe flame, it will become dark green in color.
2. Flame Color. Some zinc minerals, when a fragment is held in the forceps and heated in the reducing flame, will show a characteristic flame color. This is due to the burning in the flame of the metallic zinc which has been volatilized. It takes the form of momentary streaks or threads in the flame and has a pale greenish blue color.