ZINC 



701 



phile or sulfophile is related to the preference of an 

 element for oxygen or sulfur, respectively. Thus, 

 most lithophile elements are also oxyphile, most 

 chalcophile elements are sulfophile, and many sidero- 

 phile elements are either oxyphile, or sulfophile, or 

 weakly both. Zinc seems to be a borderline case. 

 While dominantly chalcophile, it also has some litho- 

 phile characteristics; and, although its dominantly 

 chalcophile nature would suggest it to be sulfophile, 

 it is classified as weakly oxyphile (see Rankama and 

 Sahama, 1950, tables 4.3 and 5.4). This ambiguity 

 is apparently due to its amphoteric nature — that is, 

 the habit of its hydroxide to act as a weak base as 

 well as a weak acid depending on the pH of the 

 aqueous environment. In this respect it is like alu- 

 minum, beryllium, and gallium, but unlike iron and 

 manganese. Thus, with increasing availability of 

 hydroxyl ions, a solution containing zinc as the 

 bivalent cation will precipitate Zn(0H)2 at a pH 

 of about 7-8. When the pH exceeds these values, 

 the zinc reenters the solution, but as the zincate 

 anion, Zn02~^ or the hydrated zincate anion, 

 Zn(0H)4--. The reaction is reversible; as the pH 

 of the solution decreases, the hydroxide is precipi- 

 tated once again (pH 7-8) and is then redissolved 

 as the bivalent cation with the further lowering of 

 the pH. (See Krauskopf, 1955.) 



The amounts of zinc in early magmatic sulfides are 

 low, but zinc becomes concentrated in the residual 

 solutions resulting from the continued differentia- 

 tion of a magma, and these solutions in turn con- 

 tribute to the concentrations in the pneumatolytic 

 and hydrothermal phases so important in the genesis 

 of ore deposits. On the other hand, small quantities 

 of the metal remain behind with the various crystal 

 phases of magmatic differentiation because of the 

 diadochic capability of zinc to replace ferrous iron 

 and magnesium in crystal structures. This replace- 

 ment is possible because of the identity or near 

 identity of the ionic radii of the three elements. 

 Thus, trace quantities of zinc may occur in the 

 early magmatic iron oxides, such as magnetite and 

 ilmenite. Zinc apparently does not form separate 

 silicate minerals in igneous rocks, although locally, 

 certain species may be enriched in zinc sufficiently 

 to be described as zincian varieties, (for example, 

 amphibole, pyroxene, biotite, tourmaline, and gar- 

 net) . Many of these zincian silicates, however, are 

 more likely to be found in a metamorphic regime 

 than in primary igneous rocks, such as, for example, 

 the zincian biotites at Franklin Furnace and Sterling 

 Hill, N. J. (Frondel and Einaudi, 1968). 



Most primary ore deposits are formed in a rela- 

 tively low pH environment. Consequently, most of 



the sulfophile ore metals are precipitated as sulfide 

 minerals; hence the chief primary ore mineral of 

 zinc is sphalerite, ZnS. During the weathering cycle, 

 however, sphalerite is decomposed. Most of the zinc 

 goes into solution as either the sulfate or the chlor- 

 ide and moves downward and generally away from 

 the primary deposit, to be reprecipitated as carbon- 

 ate, silicate, or other oxidized minerals depending 

 on the availability of various anions in, and the 

 chemistry of, the hydrologic regime. Zinc may be 

 thus completely leached from the upper parts of 

 primary zinc ore bodies or may be highly concen- 

 trated in the oxidized zone. Some chemical similari- 

 ties of zinc to aluminum also suggest that zinc 

 should be concentrated in bauxite. That this is not 

 true is probably due chiefly to the low Zn-Al ratios 

 in the rocks from which most commercial bauxite 

 has been formed. However, where sufficient zinc is 

 present, laterization may well result in a high-grade 

 concentration not only of zinc carbonates and sili- 

 cates but also of zinc-bearing clays and perhaps 

 zincite. 



ORE MINERALS 



Many minerals contain zinc as a major compo- 

 nent ; 55 species of zinc minerals are listed by Dana 

 (1945, p. 812). Native zinc is known but rare; Boyle 

 (1961) listed reported occurrences in his description 

 of native zinc in the oxidized zone at Keno Hill in 

 the Yukon where it occurs with other native ele- 

 ments including sulfur. He attributes its occurrence 

 there to the dissociation of sphalerite by autoreduc- 

 tion-oxidation processes. 



The principal ore mineral of zinc is sphalerite, a 

 zinc sulfide sometimes referred to as "blende" or 

 "jack." Common varieties of sphalerite are yellow 

 or resinous brown, although it may occur in other 

 colors depending upon the type and amount of im- 

 purities. The pure and nearly colorless variety is 

 known as cleiophane, whereas the dark-brown to 

 black variety with more than 10 percent Fe is 

 known as marmatite. Sphalerite crystallizes in the 

 isometric system, whereas wurtzite, a relatively 

 rare and less stable zinc sulfide, crystallizes in the 

 hexagonal system. In some deposits the ore contains 

 a banded intergrowth of sphalerite and wurtzite that 

 is known as schalenblende. Trace metals associated 

 with and recovered from zinc-sulfide concentrates 

 include silver, cadmium, germanium, gallium, in- 

 dium, and thallium. 



Zinc sulfides oxidize readily to several secondary 

 minerals, the more common of which are the zinc 

 carbonate, smithsonite, and the hydrous basic zinc 

 silicate, hemimorphite. The name "calamine" has 

 been applied sometimes to several of the oxidized 



