Halite ions

ForCb, Bt2 and h, K. c is 4.2 x 10- 7.2 x 10 and 2.0 x 10 mol 1 respectively, thereby favouring the free halogens, whereas Kaik is 7.5 X lO, 2 X lO and 30mol I respectively, indicating a tendency to disproportionation which is overwhelming for CI2 but progressively less pronounced for Br2 and I2. In actuality the situation is somewhat more complicated because of the tendency of the hypohalite ions themselves to disproportionate further to produce the corresponding halite ions ... 

The disulfide is dissolved by aqueous potassium hydroxide, yielding a greenish-yellow solution. At low temperatures no perceptible evolution of gas takes place. Since the disulfide in many respects behaves as a pseudo-halogen, Brown et al. have supposed that the reaction described by Eq. (8) takes place, i.e. a reaction analogous to the formation of halide and halite ions from a halogen and alkali. 

It remains now to consider the equilibria of the oxo anions not yet mentioned and their kinetic relations to those we have discussed. Halite ions and halous acids do not arise in the hydrolysis of the halogens. HI02 apparently does not exist, HBr02 is doubtful, while HC102 is not formed by disproportionation of ClOH if for no other reason than that the equilibrium constant is quite unfavorable ... 

D and 5 0 data on fluid inclusions and minerals, 8 C of carbonates, salinity of inclusion fluids together with the kind of host rocks indicate that the interaction of meteoric water and evolved seawater with volcanic and sedimentary rocks are important causes for the formation of ore fluids responsible for the base-metal vein-type deposits. High salinity-hydrothermal solution tends to leach hard cations (base metals, Fe, Mn) from the country rocks. Boiling may be also the cause of high salinity of base-metal ore fluids. However, this alone cannot cause very high salinity. Probably the other processes such as ion filtration by clay minerals and dissolution of halite have to be considered, but no detailed studies on these processes have been carried out. 

Af is called a formation or stability constant. Note that the formula for the ion pair, NaQ (aq), symbolizes the interaction of 1 atom of Na with 1 atom of Cl, whereas the representation of crystalline halite, NaCl(s), is an empirical formula in which an imspecified number of Na and Cl atoms are present in a 1 1 stoichiometric ratio. 

Improvements in analytical techniques have made possible reconstruction of ancient seawater composition from fluid inclusion trapped in marine halites. This has forced marine chemists to accept that the major ion composition has changed significantly— at least over the past 500 million years. Since marine halites older than 500 million years are rare, little is known about the major ion composition of seawater prior to the Phanerozoic eon. Thus, current modeling effiarts are directed at simulating changes in seawater composition over the Phanerozoic. 

A unique solution for the equilibrium concentrations of each ion is obtained by fixing the temperature and chloride concentration. The resulting atmospheric level of CO2 can also be calculated. An example of the numerical solution to this multicomponent equilibrium concentration calculation is shown in Table 21.10. The predicted major ion concentrations are close to the observed values. Nevertheless, this model is not widely accepted as realistic because little evidence has been found for the establishment of equilibria between seawater and the solid phases. In feet, concentration gradients in the bottom and pore waters suggest that equilibrium is not being attained (Figure 21.2). This model is also not able to predict chloride concentrations because the major sedimentary component (halite) is nowhere near saturation with respect to average seawater. 

The other reason why the average salinity of seawater is 35%o lies in the fundamental chemistry of major ions. For example, the sevenfold increase in the Na /K ratio between river water and seawater (Table 21.8) reflects the lower affinity of marine rocks for sodium as compared to potassium. In other words, the sodium sink is not as effective as the one for potassium. Thus, more sodium remains in seawater, with its upper limit, in theory, being controlled by the solubility of halite. Likewise, the Ca /Mg ° ratio in seawater is 12-fold lower than that of river water due to the highly effective removal of calcium through the formation of biogenic calcite. 

Why do halite crystals have such a distinct shape As we explore in this chapter, the macroscopic properties of any substance can be traced to how its submicroscopic parts are held together. The sodium and chloride ions in a halite crystal, for example, are held together in a cubic orientation, and as a result the macroscopic object we know as a halite crystal is also cubic. 

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... 

The structure of NaCl, table salt, rock salt, or halite (name of the mineral) is the most important and most common one for MX-type salts. Hundreds of salts have this structure, and many examples are listed in Table 5.1. It is characteristic of ionic MX-type compounds except for those with large differences in sizes of cations and anions. The coordination number (CN) is six for both ions for NaCl. Large cations, particularly with small anions, prefer a larger CN, commonly CN = 8 as in the CsCl structure. Small cations, and particularly for compounds with significant covalent character, prefer the CN = 4 as in ZnS. 

Halite (NaCl) can be selectively floated using n-alkyl carboxylates (collector), heavy metal ions (activator), and a non polar oil (additional collector). This yields a concentrated dispersion of the potash. Alternatively, the potash can be selectively floated from a saturated brine solution, using n-alkyl amine as the collectors, leaving the other salts behind. The flotation steps may involve a sequence of rougher, cleaner, and re-deaner stages. Either way the collected potash would be centrifuge de watered, dried, then sized by screening. The final potassium concentrate would probably be at least about 60%,... 

The concentrations of the major ions are commonly expressed in mgL they are also reported in meq or peq L , which permits a check of the ionic balance of an analysis the sum of cations (S in eq L ) should equal the sum of anions (2 in eqL ). Dissolved silica is generally not ionized at pH values commonly found in rivers its concentration is usually expressed in mgL or in pmolL . Ionic contents can also be expressed as percent of S " or S (%Q), which simphfies the determination of ionic types. Ionic ratios (C,/C ) in eq eq are also often tabulated (Na /Cl, Ca " /Mg2+, Cl /SO, etc.). As a significant fraction of sodium can be derived from atmospheric sea salt and from sedimentary halite, a chloride-corrected sodium concentration is commonly reported (Na = Na -Cl (in rneqL )). The export rate... 

It is generally agreed that most of the chloride in basinal brines has been derived from some combination of the subsurface dissolution of evaporites (e.g., Kharaka et al., 1985 Land, 1997) and the entrapment and/or infiltration of evaporated seawater (e.g.. Carpenter, 1978 Kharaka et al., 1987 Moldovanyi and Walter, 1992). Dissolution of halite produces waters dominated by sodium chloride. Evaporation of seawater produces waters having the general trends shown for ion-Br (Figure 5), Na-Cl (Figure 3) and Ca-Cl (Figure 4), but most formation waters have neither the cation (nor anion) composition of an... 

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