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ZINC Properties
Zinc is a bluish-white, rather soft metal which belongs to the Group lib elements of the Periodic Table. Zinc has atomic number 30, atomic mass 65.39, density 7.14 gjcm 3 , melting point 419.6oC and boiling point 90rC. It has five stable isotopes (64 Zn, 48.6%; 66 Zn, 27.9%; 67 Zn, 4.1%; 68 Zn, 18.8%; 70 Zn, 0.6%). The most commonly used (mainly in biological experiments) of the 18 radioactive isotopes are 65 Zn (t 112 = 245 days) and 69 Zn (t 112 =55 min). Zinc is amphoteric, it reacts both with acids and alkalis, and it is divalent in all its compounds. In aqueous solutions it exists as Zn 2 + and complex ions such as Zn(H 2 0)~+, Zn(OH)~-, Zn(NH 3 )~+, Zn(CN)~-, etc. Zn chloride, sulfate and nitrate are readily soluble in water, whereas Zn oxide, carbonate, phosphate, silicate and sulfide are practically insoluble in water. On the surface of metallic zinc a protective surface film consisting of oxide in dry air or basic carbonate (2ZnC0 3 • 3H 2 0) in wet air often forms. Zinc compounds show different coordination numbers but in oxides, sulfides, chlorides and bromides, zinc favors fourfold coordination. The effective ionic radius of Zn 2 + is 0.74 A. Uses
Zinc was discovered as metal in 1520. As for the preparation of zinc alloys (bronze, brass) is not required pure metal, so these alloys were prepared as early as ~ 1000 BC. Zinc together with lead and copper are essential metals in industry. The most important uses of zinc are anti-corrosion coating, alloys and batteries, cans, PVC stabilizers, etc. Zinc also finds uses in precipitating gold from cyanide solution, in medicines and chemicals. Geochemistry
The average concentration of zinc in Earth's crust is about 70 ppm (zinc is approximately as abundant as rubidium (78 ppm) and copper (68 ppm)). Zinc abundance in different minerals is a function of the zinc concentration in the magma
and the ability of the crystal structure to incorporate this element. It is a major constituent of more than 80 minerals, but there are only a few important commercial ores. The principal Zn ores are sphalerite (cubic ZnS) and wurtzite (hexagonal ZnS) and their weathering products, mainly smithsonite (trigonal ZnC0 3 ) and hemimorphite (rhombic Zn 4 Si 2 0 7 (0H)z · 2H 2 0). The most common impurities in zinc minerals are Fe, Cd, Pb and In. Most of the zinc deposits occur as fillings and replacements formed by low-temperature hydrothermal solutions. The unique zinc deposit is Franklin Furnace in New Jersey. The ore minerals there are zincite (ZnO), willemite (Zn 2 Si0 4 ) and franklinite (Fe,Zn,Mn)(Fe,Mn) 2 0 4 occurring as grains in calcite and high temperature and high pressure of their origin is suggested. The occurrence of zinc in some rock-forming ferrous iron and magnesium silicates and oxides (magnetite, pyroxenes, amphiboles, micas, spinel and staurolite) is far more important for the crustal abundance of this element than zinc in ore deposits. The concentration of zinc in weathering solutions is controlled rather by adsorption (on clay minerals, Fe, M
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