Quartz-feldspar-dissolution

Model the dissolution of quartz and K-feldspar (adularia) over time. Are the parameters temperature and C02 partial pressure of any importance Within the key word RATES use the BASIC program from the data set PHREEQC.dat. The calculation is done with distilled water (pH = 7, pE = 12) as a batch reaction over a time span of 10 years in 100 time steps at a temperatures of 5 °C and of 25 °C and at C02 partial pressures of 0.035 Vol% (atmosphere) and of 0.7 Vol% (soil). Calculate also the kinetics of the dissolution with 0.035 Vol% C02 and 25 °C for a period of 10 minutes. 

Using the minerals quartz and kaolinite in EQUILIBRIUM(PHASES causes a problem in PHREEQC regarding the elements Si and Al because they do not occur within the key word SOLUTION. Therefore you have to specify them in very small quantities in the solution (e.g. 1 pg/L). Furthermore, the sub key word -step divide 100 within the key word KINETICS is necessary. The output can be obtained most effectively using SELECTED OUTPUT.] 

3 Degradation of organic matter within the aquifer on reduction of redox sensitive elements (Fe, As, V, Cu, Mn, S) 

The reactions in an aquifer shall be modeled in the presence of calcite and large concentrations of pyrite and organic matter for the acid mine drainage from the exercise in chapter 3.1.3.3. As no inorganic carbon is given in the analysis and calcite is to be used as kinetically reacting mineral in the model, the analysis has to be completed by e g. 1 mg/L carbon, formally. 

PHREEQC always refers to one liter or one kg of water. The model describes a batch reaction with 1 liter water. 10 mmol calcite as well as 1 mol pyrite and 1 mol organic matter shall be present in the respective sediment/rock. To describe the kinetics of calcite and pyrite, the BASIC program given at the end of the data set PHREEQC.dat is used. For the degradation of organic matter the PHREEQC.dat notation is used, too. However, the lines 50 and 60 have to be changed as follows to accelerate the decomposition of the organic matter. Nitrate is not taken into account in this example. 

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Fig. 63 and Fig. 64 show, that both quartz and K-feldspar show different dissolution kinetics depending on temperature and C02 partial pressure. The difference between quartz and K-feldspar is significant while quartz reaches dissolution equilibrium after 150 to 550 days, K-feldspar does not show any equilibration even after 10 years for all of the four possible scenarios. To reach saturation for K-feldspar in all four models, the simulation time would have to be about 1000 years. 

Dove (1995) further summarizes evidence suggesting that adsorption of both Al " " and Fe3+ onto quartz surfaces inhibits reactivity of that phase. Inhibition of feldspar dissolution also occurs when Al is present in solution (Chou and Wollast, 1985 Nesbitt et al, 1991 Chen and Brantley, 1997). For example, Nesbitt et al. (1991) argued that adsorption of Al " " retarded the rate of dissolution of labradorite more than other cations. Furthermore, the effect of aqueous Al " " on dissolution of albite may increase with increasing temperature due to the enhanced adsorption of cations with temperature (Machesky, 1989 Chen and Brantley, 1997 note however that Oelkers (2001b) disputes this trend). In contrast, the addition of aqueous aluminum was not observed to affect the rate of forsterite dissolution at pH 3 and 65 °C (Chen and Brantley, 2000). It may be that aqueous aluminum becomes incorporated into surfaces and affects dissolution wherever the connectedness of surface silicon atoms is >0. Brantley and Stillings (1996, 1997) and Chen and Brantley (1997) suggest that Equation (51) can be used to model aluminum inhibition on feldspars. Sverdrup (1990) has reviewed the effects of aqueous Al on many minerals and incorporated these effects into rate equations. 

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

Eogenetic feldspar dissolution and kaolinization are potential sources for the early-burial quartz overgrowths. Determination of the source for quartz outgrowths and overgrowths is beyond the scope of this study. 

Diagenetic carbonate cement in reservoir sandstones of the Oseberg Formation (Brent Group) in the Oseberg field, Norwegian North Sea, occurs as disseminated siderite and ankerite, and as massively calcite-cemented intervals. Other diagenetic features include extensive feldspar dissolution and K-feldspar, quartz, kaolinite and dickite cements. Conditions of carbonate cementation are constrained on the basis of textural, geochemical and fluid inclusion evidence. 

Further conversion of mixed-fay-er clays towards illite authigenesis. Authigenic kaolinite precipitation. Authigenic chlorite development. Mixed-layer s ordering corrensite,allevardite. Feldspar dissolution.Significant quartz overgrowths development, chert dissolution. Carbonate cement dissolution/predpi-tation. Some secondary porosity development. 

Significant reduction of mixed-layer days and their further or-d ng latkb improwng crystallinity of Illite and chlorite. Emergence of dikite. Feldspar dissolution. Intens rve carbonate ament leashing (especially dolomite) and predpitation, as well as ferroan carbonates. Quartz pressure srrfution and other all-cate dissolution. Further finten-slve) porosity enhancement. 

Chemical weathering is least effective with minerals of hypogene rocks (quartz, feldspars, micas, etc.). Their dissolution is very slow and is accompanied by the formation of secondary, even less soluble hypergene minerals (clay, gibbsite, boehmite, limonite, etc.). The rate of such substitution depends on intensity of water-exchange with the surface and aggressiveness... 

Feldspar. Another ubiquitous material in crustal materials is feldspar making the study of its dissolution reaction highly relevant to understanding the weathering of continents. Concurrent with the study of quartz dissolution, Xiao and Lasaga (1996) investigated the mechanism of feldspar dissolution in acidic pH conditions. The gas phase reaction paths... 

The continental pattern for Na matches the pattern for total feldspar percentages, as Na values are primarily correlated with plagioclase (Eberl Smith 2009). Feldspars are much more susceptible to chemical dissolution than quartz and, with sufficient time and precipitation, will weather mainly to clay minerals. As a result, total feldspar contents and Na contents decrease with increasing precipitation from west to east (Fig. 3). 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

The overall rate of a fluid-rock reaction can also be modeled, rather than computing the dissolution and precipitation of each solid separately. Eor example, one could write an overall reaction between solids and fluids such as Muscovite -b Quartz = Sillimanite -b K-feldspar -b H2O. The model for overall reactions in metamorphic rocks advanced by Lasaga and Rye (1993)... 

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