Quartz dissolution rate

Fig. 27.2. Concentration of dissolved silica and the quartz dissolution rate along a quartz sand aquifer being recharged at left by rainwater, for the scenario considered in Figure 27.1. Results were calculated assuming a range of flow velocities rapid flow corresponds to a Damkohler number Da less than one, whereas Da is greater than one for slow flow.

Gautier J.-M., Oelkers E. H., and Schott J. (2001) Are quartz dissolution rates proportional to BET surface areas Geochim. Cosmochim. Acta 65(7), 1059-1070. 

Poulson S. R., Drever J. I., and Stillings L. L. (1997) Aqueous Si-oxalate complexing, oxalate adsorption onto quartz, and the effect of oxalate upon quartz dissolution rates. Chem. Geol. 140(1-2), 1-7. 

Schulz M. S. and White A. F. (1999) Chemical weathering in a tropical watershed, Luquillo mountains, Puerto Rico III. Quartz dissolution rates. Geochim. Cosmochim. Acta 63, 337-350. 

The temperature of the melt has to be high enough to provide first, a reasonable quartz dissolution rate in the molten bath and second, a manageable low melt viscosity. The latter also depends on the chosen silica to alkali ratio. Fusion temperatures of 1300-1400°C for alkaline glass and 1400-1500°C for neutral glass are common (see Table 22.1). Carbon dioxide is driven off by the reaction of alkali carbonate with silica. The main part of carbon dioxide is liberated at 700°C [1,5-7,11,12,14,15,18-21]. 

Analysis of dissolution rate as a function of temperature shows that the difference in quartz dissolution rate is greatest at low temperature, and decreases with increasing temperature. Application of the Arrhenius equation ... 

At near-neutral pH, aluminum is preferentially complexed by hydroxide, total aluminum solubility is very low, and organic-aluminum complexes are not favored. At this pH, however, organic-silica complexes are more favored, silica is mobilized, and silicate and quartz dissolution rate is accelerated. Organic-acid concentration, however, must be much higher than for the equivalent mobilization of aluminum because of the weaker silica -organic-acid interaction. In a near-neutral pH, organic-rich system, silica is mobilized from the silicate surface with the released aluminum unstable with... 

Fig. 16.1. Results of reacting quartz sand at 100°C with deionized water, calculated according to a kinetic rate law. Top diagram shows how the saturation state Q/K of quartz varies with time bottom plot shows change in amount (mmol) of quartz in system (bold line). The slope of the tangent to the curve (fine line) is the instantaneous reaction rate, the negative of the dissolution rate, shown at one day of reaction.

We can confirm that on a plot of the mole number nqtz for quartz versus time (Fig. 16.1), this value is the slope of the tangent line and hence the dissolution rate —dwqtz/dt we expect. 

In the calculation results (Fig. 26.6), the initial segment of the path is marked by the disappearance of the amorphous silica as it reacts to form cristobalite. The amorphous silica is almost completely consumed after about 10000 years of reaction. The mineral s mass approaches zero asymptotically because (as can be seen in Equation 26.1) as its surface area As decreases, the dissolution rate slows proportionately. During the initial period, only a small amount of quartz forms. 

We set quartz dissolution and precipitation according to a kinetic rate law (Knauss and Wolery, 1988 see Chapter 16),... 

The dissolution of quartz is accelerated by bi- or multidentate ligands such as oxalate or citrate at neutral pH-values. The effect is due to surface complex formation of these ligands to the Si02-surface (Bennett, 1991). In the higher pH-range the dissolution of quartz is increased by alkali cations (Bennett, 1991). Most likely these cations can form inner-spheric complexes with the =SiO groups. Such a complex formation is accompanied by a deprotonation of the oxygen atoms in the surface lattice (see Examples 2.4 and 5.1). This increase in C H leads to an increase in dissolution rate (see Fig. 5.9c). 

Convective dissolution rate for quartz in an andesitic melt may be calculated similarly, but the error may be larger than the normal 20% relative because quartz dissolution increases Si02 content so much, leading to orders of magnitude increase in viscosity for the interface melt (viscosity is about 120 Pa s for the initial andesitic melt and 1.7 x lO" Pa s for the interface rhyolitic melt). Because... 

Mechanisms and rates of quartz dissolution in melts in the CMAS (CaO-... 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

The removal of silica from a siliceous iron ore, such as the taconites found in Minnesota and Wisconsin, has been studied by Tiemann (T7, T9). Caustic concentrations from 25-500 gm/liter were used to digest the ore in a bomb at temperatures from 312 to 408°F. The leaching pressures in the bomb correspond closely to the equilibrium vapor pressures of the sodium hydroxide solutions used. A residual concentrate containing around 65% iron was obtained with —200 mesh material in 60 min of contact time. The high rate of dissolution of the silica was attributed to its occurrence in the form of microcrystalline (chalcedonic) varieties with high specific surface. The dissolution rate of pure quartz is directly proportional to the surface area and an average rate of 17 X 10 gm moles/cm sec was obtained for a 100 gm/liter NaOH solution at 312°F for the —400 mesh fraction. 

Figure 4 Log (dissolution rate) versus pH for quartz at 25 °C measured in various pH buffers in agitated batch reactors. The slope of the log rate — pH curve equals 0.3 above the pristine point of zero charge (Brady and Walther, 1992) (source Brady and Walther, 1990).

The most comprehensive dataset for a silicate has been compiled for quartz dissolution over a range of temperature and pressure (e.g., Rimstidt and Barnes, 1980 Knauss and Wolery, 1988 Wollast and Chou, 1988 Brady and Walther, 1990 Dove and Crerar, 1990 Hiemstra and van Riemsdijk, 1990 Dove and Elston, 1992 Dove and Rimstidt, 1994 Tester et al., 1994 Dove, 1995). Dove (1994, 1995) has modelled the rate of dissolution of quartz as a function of temperature by the rate equation,... 

Figure 5 Log (dissolution rate) of quartz (mol m s ) plotted as a function of solution pH and dissolved sodium concentration at 25 °C and 200 °C as predicted by Equation (30) (reproduced by permission of Am. J. Sci. 1994, 294, 665-712).

The effect of organic anions on silicates other than quartz and feldspar has also been investigated. Dissolution rates of kaolinite are enhanced more by oxalate than salicylate, while malonate and phthalate show little effect (CarroU-Webb and Walther, 1988 Chin and Mills, 1991 Wieland and Stumm, 1992). Dissolution of hornblende is accelerated at pH 4 in the presence of organic acids at 2.5 mM rate enhancement was observed... 

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