Zinc , wurtzite sphalerite

For compounds of the composition MX (M = cation, X = anion) the CsCl type has the largest Madelung constant. In this structure type a Cs+ ion is in contact with eight Cl-ions in a cubic arrangement (Fig. 7.1). The Cl- ions have no contact with one another. With cations smaller than Cs+ the Cl- ions come closer together and when the radius ratio has the value of rM/rx = 0.732, the Cl- ions are in contact with each other. When rM/rx < 0.732, the Cl- ions remain in contact, but there is no more contact between anions and cations. Now another structure type is favored its Madelung constant is indeed smaller, but it again allows contact of cations with anions. This is achieved by the smaller coordination number 6 of the ions that is fulfilled in the NaCl type (Fig. 7.1). When the radius ratio becomes even smaller, the zinc blende (sphalerite) or the wurtzite type should occur, in which the ions only have the coordination number 4 (Fig. 7.1 zinc blende and wurtzite are two modifications of ZnS). 

By substituting alternately the carbon atoms in cubic diamond by zinc and sulfur atoms, one obtains the structure of zinc blende (sphalerite). By the corresponding substitution in hexagonal diamond, the wurtzite structure results. As long as atoms of one element are allowed to be bonded only to atoms of the other element, binary compounds can only have a 1 1 composition. For the four bonds per atom an average of four electrons per atom are needed this condition is fulfilled if the total number of valence electrons is four times the number of atoms. Possible element combinations and examples are given in Table 12.1. 

Structure of cubic (left) and hexagonal (right) diamond. Top row connected layers as in a-As. Central row the same layers in projection perpendicular to the layers. Bottom unit cells when the light and dark atoms are different, this corresponds to the structures of zinc blende (sphalerite) and wurtzite, respectively... 

Wurtzite structure. Zinc sulfide can also crystallize in a hexagonal form called wurtzite that is formed slightly less exothermically than the cubic zinc blende (sphalerite) modification (Afff = —192.6 and —206.0 kJ mol-1, respectively) and hence is a high temperature polymorph of ZnS. The relationship between the two structures is best described in terms of close packing (Section 4.3) in zinc blende, the anions (or cations) form a cubic close-packed array, whereas in wurtzite they form hexagonal close-packed arrays. This relationship is illustrated in Fig. 4.13 note, however, that this does not represent the actual unit cell of either form. 

N = NaCl structure W = wurtzite structure Z = zinc-blende (sphalerite) structure... 

Figure 7.22 Three-dimensional nets (a) the cubic diamond structure (b) the net equivalent to (a) (c) the cubic zinc blende (sphalerite) structure (d) the net equivalent to (c), which is identical to that in (b) (e) the hexagonal wurtzite structure (f) the net equivalent to (e)...

The structure of both the SiC and ZnS polytypes can be illustrated with reference to the crystalline forms of ZnS. Zinc sulphide crystallises in either of two structures, one of which is cubic and given the mineral name zinc blende (sphalerite) while the other is hexagonal and given the mineral name wurtzite. The relationship... 

Silicon carbide, carborundum, also crystallises in two forms, of which /(-SiC has the cubic zinc blende (sphalerite) structure (Figure 8.8a). When viewed along the cube face-diagonal [110] direction, the layers of both silicon and carbon are packed in the cubic closest packing arrangement. .. aAbBcCaAbBcC. .., where the uppercase and lowercase letters stand for layers of Si and C. The other form of silicon carbide, a-SiC, is a collective name for the various silicon carbide polytypes, which consist of complex arrangements of zinc blende and wurtzite slabs. Some of these are known by names such as carborundum I, carborundum II, carborundum III, and so on. One of the simplest structures is that of carbo-... 

Zinc sulfide (sphalerite) ZnS 1314-98-3 97.474 gray-wh cub cry trans wurtzite 1020 4.04 iHjO, EtOH s dil acid... 

Figure 32. The wurtzite type structure of a-BeO, space group Ptjmc. The three-dimensional network of condensed Be04-tetrabedra is outlined. The stacking of these tetrahedra has the sequence AB, AB. The corresponding stacking ABC, ABC is known to occur for the zinc blende (sphalerite) structure (Fig. 3).

Table 2 Elemental combinations with an average of four valence electrons per atom that, accordingly, crystallize in the zinc blende (sphalerite) or wurtzite structural types.

Matraite is a zinc sulfide mineral with composition ZnS. It was first described by Koch in 1958 from Gyongyosoroszi (Matra Mountains, Hungary) and is still only known from this locality. It occurs as aggregates of brown to brown-yellow anhedral or hemimorphic pyramidal crystals. It is a rare mineral in nature the iron-bearing varieties of zinc sulfide, sphalerite and wurtzite (qq.v.), are more commonly encountered. 

For sphalerite and wurtzite, for example, the discussion of partial ionic character as described above for molyde-nite leads to the resultant average charges +0.67 for sulfur and—0.67 for zinc. The distribution of the sulfur atoms is calculated to be 12% S2 (quadricovalent), 50 percent S+, 32 percent S°, 6 percent S-, 0.2% S2-. The observed bond length 2.34 A with the sulfur radius 1.03 A and the Schomaker-Stevenson correction 0.05 A leads to 5 = 1.36 A for zinc (quadricovalent Zn2-). The increase by 0.05 A over the value 1.309 A for sp3 bonds of Zn° is reasonable as the result of screening of the nucleus by the extra electrons. 

Fig. 1.2 Crystal structures of the major sulfides (metal atoms are shown as smaller or black spheres) (A) galena (PbS) structure (rock salt) (B) sphalerite (ZnS) structure (zinc blende) (C) wurtzite (ZnS) strucmre (D) pyrite structure and the linkage of metal-sulfur octahedra along the c-axis direction in (/) pyrite (FeSa) and (//) marcasite (FeSa) (E) niccolite (NiAs) structure (F) coveUite (CuS) structure (layered). (Adapted from Vaughan DJ (2005) Sulphides. In Selley RC, Robin L, Cocks M, Plimer IR (eds.) Encyclopedia of Geology, MINERALS, Elsevier p 574 (doi 10.1016/B0-12-369396-9/00276-8))...

Zinc sulfide, ZnS, sphalerite (zinc blende) zinc sulfide, ZnS, wurtzite zinc selenide, ZnSe zinc telluride, ZnTe, cubic zinc telluride, ZnTe, hexagonal zinc polonide, ZnPo zinc aluminum selenide, ZnAl2Se4 zinc indium selenide, ZnIn2Se4 zinc indium telluride, Znhi2Te4. 

World production of zinc increased from 0.5 million metric tons in 1900 to 6.1 million tons in 1978 (Elinder 1986) to 7.1 million tons in 1987 (USPHS 1989). The principal ores of zinc are sulfides, such as sphalerite and wurtzite (Elinder 1986). The major world producers include Canada, the former Soviet Union, and Japan — which collectively account for about half the production — and, secondly, the United States, Australasia, Mexico, and Peru (Weatherley et al. 1980 Elinder 1986). Zinc is now available as ingots, lumps, sheets, wire, shot, strips, granules, and powder (USPHS 1989). The United States produced 240,000 tons of zinc in 1987 — mostly from Tennessee, Mississippi, and New York, but also 16 other states — and imported an additional 774,000 metric tons, thus consuming 14% of world zinc production while producing 3.4% (USPHS 1989). 

Some of the discharged sulfide particles settle onto the chimney s exterior, where they are buried by the outward growth of anhydrite. Sulfide precipitation within the chimneys, causes copper, zinc, and iron sulfides to deposit and partially replace the anhydrite. Chimneys can build to several meters in height and their orifices range in diameter from 1 to 30 cm. Both the smoke and the chimneys are composed of polymetallic sulfide minerals, chiefly pyrrhotite (FeS), pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite or wurtzite (ZnS). 

It is not found in its pure metallic form in nature but is refined from the mineral (compound) zinc sulfide (ZnSO ) known as the ores sphalerite and zincblende. It is also recovered from minerals and ores known as willemite, hydrozincite, smithsonite, wurtzite, zincite, and Franklinite. Zinc ores are found in Canada, Mexico, Australia, and Belgium, as well as in the United States. Valuable grades of zinc ores are mined in Colorado and New Jersey. 

Zinc occurs in nature, widely distributed. The principal ores are sphalerite (and wurtzite) known as zinc blende, ZnS gahnite, ZnAl204 calamine smith-sonite, ZnCOs franklinite, ZnFe204 and zincite, ZnO. Abundance in earth s crust is about 70 mg/kg and average concentration in sea water is about 10 pg/L. 

Zinc sulfide occurs in nature in two crystalline forms, the minerals, wurtzite, and sphalerite. Sulfide ore is the principal zinc mineral. 

Zinc sulfide is white to gray-white or pale yellow powder. It exists in two crystalline forms, an alpha (wurtzite) and a beta (sphalerite). The wurtzite form has hexagonal crystal structure refractive index 2.356 density 3.98 g/cm3 melts at 1,700°C practically insoluble in water, about 6.9 mg/L insoluble in alkalis soluble in mineral acids. The sphalerite form arranges in cubic crystalline state refractive index 2.368 density 4.102 g/cm changes to alpha form at 1,020°C practically insoluble in water, 6.5 mg/L soluble in mineral... 

Sphalerite and wurtzite, the two common forms of zinc sulfide,68 have the tetrahedral structures shown in Figures 7-5 and 7-6. Pure zinc sulfide is colorless. The minerals are usually yellow, brown, or black, the color probably being due to imperfections and impurities. The luster is not metallic, but resinous or adamantine. 

Many sulfide minerals have structures closely related to those of sphalerite and wurtzite. Chalcopyrite, CuFeS, is an example (Fig. 11-21). Its structure 0 is a tetragonal superstructure of sphalerite, with the copper and iron atoms in the zinc positions of sphalerite. 

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