Anion and cation bonding strengths

Fig. 4.6. Known silicates plotted as a function of anion and cation bonding strengths. The lines correspond to the sjsn ratios of 2.0, 1.0, and 0.5.

Examination of Figure 1-12 provides some clue to qualitatively gauge the interface reaction rate for reactions in water. Figure 1-12 shows that, for mineral with low solubility and high bond strength (characterized by (z+z )max, where z+ and z are valences of ions to be dissociated), the overall dissolution rate is controlled by interface reaction otherwise, it is controlled by mass transport. Because diffusivities of common cations and anions in water do not differ much (by less than a factor of 10 Table l-3a), when the overall reaction rate is controlled by interface reaction, it means that interface reaction is slow when the overall reaction rate is controlled by mass transport, the interface reaction rate is rapid. Therefore, from Figure 1-12, we may conclude that the interface reaction rate increases with mineral solubility and decreases with bond strength (z+z )max to be dissociated. 

The bonding strengths of several cations and anions are compared in Fig. 4.2 which shows which cations and anions have similar bonding strengths and thus form stable compounds. For example, Mg + and SiO both form bonds with valences of 0.33 vu and so readily bond to each other to form Mg2Si04 (26374), the mineral forsterite believed to form a large portion of the earth s mantle. Similarly,... 

One consequence of the ability of Tl to vary its electronic configuration in response to its environment is that it can adopt a cation bonding strength anywhere in the range from 0.11 to 0.33 vu. Unlike the similar sized but hard Rb+ ion, which only forms stable bonds with weak anions, TU is soft and can form stable compounds with both strong and weak anions (Section 4.5). 

Table A4.2 Bonding strengths for mixed anions and cations...

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