Silica saturation

The production of copper from sulphide minerals is accomplished with a preliminary partial roast of die sulphides before reaction widr air in the liquid state, known as mattes, to form copper metal (conversion). The principal sources of copper are minerals such as chalcopyrite, CuFeSa and bornite CuaFeSa, and hence the conversion process must accomplish the preferential oxidation of non, in the form of FeO, before the copper metal appears. As mentioned before, tire FeO-SiOa liquid system is practically Raoultian, and so it is relatively easy to calculate the amount of iron oxidation which can be canned out to form this liquid slag as a function of the FeO/SiOa ratio before copper oxidation occurs. The liquid slag has a maximum mole fraction of FeO at the matte blowing temperatures of about 0.3, at solid silica saturation. 

Fig. 13. Quartz ai>d amorphous silica solubility vs. temperature along the vapour saturation curve. The dashed lines show the silica concentration in water initially in equilibrium with quartz during adiabatic boiling to 100 C and subsequent cooling. The increase in aqueous silica concentrations during boiling is the consequence of steam formation. Amorphous silica saturation (shown by the dots) is attained at 188 C in the case of the 300 C aquifer water, but at 94 C in the case of the 200 C aquircr water. It was assumed that the pH of the water is not raised sufficiently during boiling to cause significant ionization of the aqueous silica. If some ionization had occurred, amorphous silica saturation would be reached at lower temperatures than those indicated in Fig. 13.

Fig. 3. Plot of logio normalized ion-exchange rate at amorphous silica saturation vs. the amount of excess alkalis (Na, K), denoted by the molar ratio XAlk/(Al + IVB + FeT). All boron is treated as four-fold coordinated (IVB) and total iron (FeT) is regarded as ferric. The ion-exchange rate subtracts out the contribution of alkalis to solution from matrix dissolution. As the amount of excess alkali increases, the ion-exchange rate increases. This increase in rate reflects the increasing amount of alkalis in non-bridging oxygen (NBO) configurations. Error bars represent 2-

Finally, our observations regarding the longterm impact of alkali ion exchange on glass dissolution now provide a mechanistic basis for the empirical residual rate of reaction appended to the TST rate law articulated by Grambow (1985). The residual rate was appended to prevent calculated glass dissolution rates from dropping to zero under silica-saturated conditions, which is not in accord with experimental observations. 

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

The source of the large amounts of silica present in Precambrian cherts is unclear. Since few siliceous organisms (Klemm, 1979)8> have been reported to occur so early in sediments, it is assumed that most or all of the cherts were of non-biological origin and derived as precipitates from silica-saturated... 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

Figure 5.10 CP/MAS 29Si NMR spectrum of Fisher S-157 silica saturated with liquid water taken from ref. (21) with permission.

Nicholls 1. A. and Ringwood A. E. (1973) Effect of water on olivine stability in tholeiites and the production of silica-saturated magmas in the island-arc environment. J. Geol. 81, 285-300. 

Although this idea has now been largely superceded by the polybaric melting model a new possibility has recently emerged whereby the uppermost mantle may behave as a chemical filter. Midocean ridge basalt (MORB) is silica saturated and is in equilibrium with orthopyroxene at pressures greater than about 8 kb. At the lower pressures commensurate with the base of the oceanic crust MORB melts are undersaturated in silica and have the capacity to dissolve orthopyroxene (Braun Kelemen, 2002). There is field evidence from... 

Perhaps least well systematized are the occurrences of zeolites in volcanic rocks and breccias, the source of so many handsome show-case specimens. Walker (44, 45) has described a low-temperature depth zona-tion transecting volcanic stratigraphy in the lava piles of Northern Ireland and Iceland crude correlation between zeolite species and silica-saturation of host lavas can also be detected (10). 

Figure 23. Fc sorption on synthetic single quartz epi quality surfaces. GI-EXAFS Fourier transform functions. Bottom two functions show effect of sorption from silica-saturated solution, hor 90 refers to electric vector in the plane of the quartz surface at right angles to the mirror plane, vert is e-vector perpendicular to the surface. Middle two functions contrast analogous experiment done without silica saturation. Top two functions show the effect of diying (i.e., ex situ experiment) on the unsaturated sorption sample. The diy and washed ex situ sample has been cleaned with a high pressure jet of DI water to remove surface precipitates. It s function is consistent with soibed complexes with little precipitate signature.

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