Aluminosilicate dissolution reactions

Oelkers, E. H., Schott, J. Devidal, J. L. (1994). The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochimica et Cosmochimica Acta, 58, 2011-24. 

The concentrations of dissolved species in natnral waters depend ultimately on the dissolution of basic rocks-carbonates, silicates and aluminosilicates-induced by the action of weak acids in the water derived from dissolved gases-e.g. H2CO3 derived from CO2. Anions produced in acid-base reactions balance cations produced in dissolution reactions. The charge balance is ... 

Table 2.2, half of the carhon source for HCO3 comes from CO2 and half from CaCOs. Comhination of these two mass balance concepts requires that about half of the atmospheric CO2 source be involved in CaCOs dissolution reactions and the other half be consumed by dissolution of aluminosilicates or, about half of the HCO3 in river water that has an atmospheric origin has reacted with aluminosilicates (Fig. 2.2). 

The solid phase of soils can consist of many Al-bearing materials, including organic matter oxides and hydroxides of Al noncrystalline aluminosilicates layer silicate clays and various primary minerals. Any model of Al solubility that is based on the dissolution reaction of only one of these materials is likely to be too simple. Nevertheless, some of these models will now be considered in turn. 

A complete description of any groundwater system necessitates consideration of reactions between rock forming minerals and the aqueous phase. This cannot be achieved without accurate thermodynamic properties of both the participating aluminosilicate minerals and aqueous aluminum species. Most computer codes used to calculate the distribution of species in the aqueous phase utilize the "reaction constant" approach as opposed to the "Gibbs free energy minimization" approach (3). In the former, aluminosilicate dissolution constants are usually written in terms of the aqueous aluminum species, Al, which is related to other aqueous aluminum species by appropriate dissociation reactions. 

Several porosity-destroying reactions also characterize this zone. For example, late ferroan carbonate cements are potential reaction products in this zone, and may significantly occlude porosity (Boles 1978). Also, there is a wide variety of aluminosilicate reaction products (e.g., kaolinite, illite, chlorite, and quartz) that can form in this zone as a result of aluminosilicate dissolution. The imbalance between porosity-enhancing or-preserving reac-... 

The heats of reaction/solution of some reagents as hydrolysis/dissolution takes place can cause substantial elevation in slurry/solution temperatures, particularly at a large scale where heat transfer and radiative cooUng are not nearly as efficient as it is in small laboratory vessels. Other reagents, such as certain sodium alumi-nates and particularly reagents that are not freshly prepared, may need elevated temperatures for full dissolution in water. These hot or very warm solutions can adversely affect early nucleation conditions in some zeoHte syntheses. Hot reagent solutions and mixtures are sometimes cooled prior to their addition to other reagents to better control the early reactions and speciation of aluminosilicate and silicate precursors. 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

Reaction 6.16 results in the pH of the waters, in the absence of carbonate precipitation, being buffered at higher pH values (e.g., Ben-Yaakov, 1973). It is, therefore, reasonable to expect that the effectiveness of sulfate reduction in producing carbonate dissolution or precipitation may depend in part on the availability of reactive iron. This conclusion has been demonstrated in a study of the pore water geochemistry of aluminosilicate and carbonate-rich sediments from Kaneohe Bay, Hawaii. The aluminosilicate sediments contain abundant pyrite whereas the pyrite content of the carbonate sediments in this bay is low. Pore waters collected from the aluminosilicate sediments have higher pH values than those collected from the carbonate-rich sediments. This observation is a result of the pH values in the pore waters of the aluminosilicate sediments being buffered at... 

In contrast to the partial pressure, temperature rise does not generally contribute to the increase of the solubility. According to the principle of the smallest constraint (Le Chatelier), only endothermic dissolutions, i.e. reactions, which need additional heat, are favored (e.g. dissolution of silicates, aluminosilicates, oxides, etc.). Yet the dissolution of carbonates and sulfates is an exothermic reaction. Therefore the solubility of carbonates and sulfates is less favorable with increasing temperature. 

Reactions M-O represent incongruent dissolution of Ca(OH)2s because the solubility of CaCOjS is much smaller than the solubility of Ca(OH)2s (Table 2.7). Therefore, introduction of Ca(OH)2s to water in equilibrium with atmospheric C02 leads to spontaneous formation of CaC03s. The well-known incongruent dissolution phenomena are those representing the dissolution of aluminosilicate minerals. For example, K-feldspars (orthoclase) undergo incongruent dissolution when exposed to water and carbonic acid to form kaolinite ... 

The dominance of carbonate hydrolysis, carbo-nation, and sulfide oxidation in subglacial weathering reactions on aluminosilicate/silicate bedrock is also found on carbonate bedrock. However, the balance between carbonate dissolution and sulfide oxidation depends on the spatial distribution of sulfides in the bedrock and basal debris (Fairchild et al., 1999). Noncongruent dissolution of strontium and magnesium from carbonate is also observed in high rock water weathering environments, such as the distributed drainage systems, in which water flow is also low (Fairchild et al., 1999). 

The question remains as to what processes cause the kinetics of opal dissolution to change as the mineral ages in marine sediments. A striking clue to the answer was the observation that the asymptotic H4Si04 pore-water values in Southern Ocean sediments are strongly dependent on the amount of detrital material present in the opal-rich sediments (Figure 17). Since earher studies had established that aluminum is reactive in marine sediments (Mackin and Aller, 1984), this correlation implies that detrital aluminosilicates supply the aluminum necessary for reactions that take place very soon after deposition. 

JLm diameter spheres, called lepispheres, which are composed of bladed crystals 30-50 nm thick. Kastner et al. (1977) reported that the formation of opal-CT results from dissolution followed by reprecipitation reactions that require a source of magnesium, alkalinity, and hydroxide ion. The presence of aluminosilicate phases commonly retards the formation of opal-CT (Hinman, 1998). 

The acid generated through sulfide oxidation reacts with the nonsulfide gangue minerals within the mine wastes. The most significant pH-buffering reactions in mine settings are the dissolution of carbonate minerals, aluminum hydroxide and ferric oxyhydroxide minerals, and aluminosilicate minerals. 

In cases where neutral or alkaline mine drainage predominates, problems may arise because of elevated concentrations of SO4, iron, manganese, and other solutes that are derived from sulfide oxidation or from reactions with carbonate or aluminosilicate minerals. Dissolved iron and aluminum may precipitate as the pH increases, and these precipitates can act as substrates for adsorption and co-precipitation (Stumm and Sulzberger, 1992 Foos, 1997 Brake et al., 2001). The dissolution of siderite,... 

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