Halite crystal structure

Commonly, many compounds with the same formula type have the same type of structure. Usually the most common compound is chosen as representative, but it could be the first structure of the type studied. For example, hundreds of binary compounds of the MX type have the same crystal structure as NaCl. Other compounds can be described as having the NaCl (or halite, the name of the mineral) structure. If the structure being considered has slight differences, these differences can be described in terms of the reference structure. One often sees statements such as a compound has a disordered spinel (MgAl204) type structure or an inverse spinel structure. This requires knowledge of the spinel structure because "inverse" or "disordered" terms describe variations of occupancies of octahedral and tetrahedral sites. 

The differences for gypsum and calcite are consistent with considerations on the question of dependence of hardness on structure and bond forces in a crystal (cf. Chapter 3). Thus, when plotting the curve for hardness scatter ranges, both gypsum and calcite have been omitted. Halite, too, has been left out on account of its considerable plasticity under load. 

Many simple minerals, especially simple salts like halite, NaCl, sulfides, sulfosalts and oxides, have structures based upon cubic or hexagonal closest-packed arrays of either cations or anions. Coordination geometries of metal ions in many of these kinds of minerals are thus confined to more or less regular octahedra and tetrahedra. The occupancy of the two types of sites is dictated by the stoichiometry of the mineral, the radius of the ions involved and their preferred coordination geometries. Coordination of cations in mineral species in terms of bonding and crystal field effects has been extensively reviewed.16-21 Comprehensive lists of ionic radii relevant to cation coordination geometries in minerals have also been compiled.16,21... 

When prepared from mixed oxides, chemical homogeneity in these materials is hard to achieve. Under these circumstances, a variety of different n-values can be said to coincide in a crystal. The defects here are regarded as slabs of the halite type interspersed more or less at random in the perovskite-structure matrix. 

FCj O or wiistite has probably the most studied defect structure. The hypothetical compound where x = 0 would crystallize with a perfect version of the halite structure where iron lies in an octahedral site surrounded by oxygen atoms. Removal of some of the iron should just give iron vacancies with the remaining irons remaining in an octahedral environment. 

TiO, where 0.7 < x < 1.25, crystallizes with the sodium chloride structure. By consideration of the incorporation of vacancies into the halite structure, describe how defects could be incorporated to give the formulae at the extremes of the values of x. 

Sodium chloride forms crystals with cubic symmetry. In these, the larger chloride ions are arranged in a cubic close-packing, while the smaller sodium ions fdl the octahedral gaps between them. Each ion is surrounded by six of the other kind. This same basic structure is found in many other minerals, and is known as the halite structure. 

The general formula of crystals with the halite structure is MX. The mineral halite, which names the group, is sodium chloride, NaCl, also called rock salt. 

Systematic absences arise from symmetry considerations and always have F(hkl) equal to zero. They are quite different from structural absences, which arise because the scattering factors of the atoms combine so as to give a value of F(hkl) = zero for other reasons. For example, the (100) diffraction spots in NaCl and KC1 are systematically absent, as the crystals adopt the halite structure, which is derived from an all-face centred (F) lattice, (see Table 6.4). On the other hand, the (111) reflection is present in NaCl, but is (virtually) absent in KC1 for structural reasons - the atomic scattering factor for K+ is virtually equal to that of Cl-, as the number of electrons on both ions is 18. 

Should the anions adopt hexagonal close-packing and all of the octahedral sites contain a cation, the hexagonal analogue of the halite structure is produced. In this case, the formula of the crystal is again MX. The structure is the nicolite, (NiAs), structure, and is adopted by a number of alloys and metallic sulphides, including NiAs, CoS, VS, FeS and TiS. 

Model A Assume that the iron atoms in the crystal are in a perfect array, identical to the metal atoms in the halite structure and an excess of oxygen is due to interstitial oxygen atoms over and above those on the normal anion positions. The ideal unit cell of the structure contains 4 Fe and 4 O, and so, in this model, the unit cell must contain 4 atoms of Fe and 4(1+x) atoms of oxygen. The unit cell contents are Fc404+4a and the composition is FeOi.059-... 

The components making up the fine-grained fractions of the terrigenous-halitic complex are essentially Mg-rich trioctahedral well-crystallized chlorites without swelling layers as well as Fe-illites, the structure of which does not contain swelling layers (see model in Fig. 2.10). At the same time there are no mixed-layer species of the chlorite-montmorillonite type which were so characteristic of the carbonate-terrigenous complex (dolomite-sulfate facies). Mixed-layer clays of the illite-montmoriUonite type also diminish throughout the complex. 

The unit cell size of CaO is 0.48105 nm, and that of SrO is 0.51602 nm. Both adopt the halite structure type. Estimate the composition of a crystal of formula Ca cSri cO, which was found to have a unit cell of 0.5003 nm. 

The preferred slip plane in ionic crystals with the halite (NaCl) structure, such as NaCl or LiF, is 110, and the slip direction used is (110). This slip system is sketched in Figure 10.17. For the more metallic halite structure solids such as titanium carbide (TiC), the slip system is similar to that in face-centred cubic metals, 1 1 1 (110). 

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