Gypsum dissociate

The single-ion activity coefficient (y) for Ca2+ or S04 for a system in equilibrium with gypsum is approximately 0.5 (see the section entitled Iteration Example in this chapter). Therefore, dissociated Ca2+ or SO concentration equals... 

Under a normal atmosphere, however, the anions are dissociated by oxygen and carbon dioxide, and precipitation of sulfur continues in the manner described, together with some calcite and gypsum. 

For instance, calcite solubility increases in the presence of CO due to the formation of H COj and HCOj, and of silicon - due to increase in pH because of dissociation of orthosilicic add H SiO (see below). Figure 2.29 shows correlation of gypsum effective solubility vs. various electrolytes. 

Minerals with the Diffuse Dissolution Condition To this group belong minerals with relatively simple dissolution mechanism (hydration and dissociation) and with solubihty much higher that 10" mole l". Their total max dissolution rate is usually no lower than 0.01 mole-m s" and little depend on water temperature and pH. Their solubility value reaches 7 mole-1 F Examples of such salts are sylvinite, soda, bischofite, etc. (Table 2.20) but most common among them are, without a doubt, halite and gypsum. 

Gypsum has structure of alternating layers of electrostatically associated ions Ca and and layers of dipoles H O. H O bonds are very weak and are easily destroyed. That is why gypsum dissolution mechanism is controlled mostly by the dissociation reaction... 

Cave Hill Cemetery, Louisville. Angels showing exfolliation of gypsum crust in areas under the dome and grain by grain dissociation of marble exposed to direct impact of rain. 

For purposes of illustration we will assume complete dissociation of the gypsum. The system may then be described water dissociation... 

Since complete dissociation is being assumed for the gypsum and sodium chloride, the possible formation of complexes such as NaSO and HSOh is disregarded. Given the initial moles of water, HaOfin). gypsum as moles of CaSO. CaSOndn), and sodium chloride. NaCl(in), in the solution, the model must determine nine unknowns ... 

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