Minerals, dissolution

Berner, R. A. (1978). Rate control of mineral dissolution under earth surface conditions. Am.. Sci. 278, 1235-1252. 

Casey, W. H. and Ludwig, C. (1995). Silicate mineral dissolution as a ligand-exchange reaction. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America Washington, DC, Reviews in Mineralogy 31, 87-117. 

The latter two assumptions are simplistic, considering the number of factors that affect pH and oxidation state in the oceans (e.g., Sillen, 1967 Holland, 1978 McDuff and Morel, 1980). Consumption and production of CO2 and O2 by plant and animal life, reactions among silicate minerals, dissolution and precipitation of carbonate minerals, solute fluxes from rivers, and reaction between convecting seawater and oceanic crust all affect these variables. Nonetheless, it will be interesting to compare the results of this simple calculation to observation. 

The reaction rate Rj in these equations is a catch-all for the many types of reactions by which a component can be added to or removed from solution in a geochemical model. It is the sum of the effects of equilibrium reactions, such as dissolution and precipitation of buffer minerals and the sorption and desorption of species on mineral surfaces, as well as the kinetics of mineral dissolution and precipitation reactions, redox reactions, and microbial activity. 

It is further interesting to observe that the behavior of a system approaching a thermodynamic equilibrium differs little from one approaching a steady state. According to the kinetic interpretation of equilibrium, as discussed in Chapter 16, a mineral is saturated in a fluid when it precipitates and dissolves at equal rates. At a steady state, similarly, the net rate at which a component is consumed by the precipitation reactions of two or more minerals balances with the net rate at which it is produced by the minerals dissolution reactions. Thermodynamic equilibrium viewed from the perspective of kinetic theory, therefore, is a special case of the steady state. 

In this chapter, we build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

As in the previous simulation, the calculation results assume a stationary state (Fig. 27.3) at which the rates of mineral dissolution and precipitation at any point in the profile are nearly invariant in time. 

ABSTRACT Atmospheric carbon dioxide is trapped within magnesium carbonate minerals during mining and processing of ultramafic-hosted ore. The extent of mineral-fluid reaction is consistent with laboratory experiments on the rates of mineral dissolution. Incorporation of new serpentine dissolution kinetic rate laws into geochemical models for carbon storage in ultramafic-hosted aquifers may therefore improve predictions of the rates of carbon mineralization in the subsurface. 

KEYWORDS carbon sequestration, tailings, serpentinite, mineral dissolution, weathering... 

Mineral dissolution, 26 6-1 in stream water, 26 25 Mineral dissolution reactions in stream water, 26 25 Mineral dressing, 16 128 Mineral feedstocks, titanium dioxide, 25 31-33... 

Bruno, J., W. Stumm, P. Wersin, and F. Brandberg (1991), The Influence of Carbonate in Mineral Dissolution. Part 1, The Thermodynamics and Kinetics of Hematite Dissolution in Bicarbonate Solution at T = 25° C", in preparation. 

Wehrli, B. (1989), "Monte Carlo Simulations of Surface Morphologies During Mineral Dissolution", J. Coll. Int. Sd. 132, 230-242. 

A common phenomenon in the dissolution of silicate minerals is the formation of etch pits at the surface (90-91.,93-94). When this occurs, the overall rate of mineral dissolution is non-uniform, and dissolution occurs preferentially at dislocations or defects that intercept the crystal surface. Preferential dissolution of the mineral could explain why surface spectroscopic studies have failed... 

The importance of "parabolic kinetics" in laboratory studies of mineral dissolution has varied as interpretations of the underlying rate-controlling mechanism have changed. Much of the research on silicate mineral weathering undertaken in the past decade or so served to test various hypotheses for the origin of parabolic kinetics. 

The weathering of minerals forms particles with a size continuum from ions to grains. Mineral dissolution and precipitation occur more or less continuously as a function of ambient conditions. Particles of the clay textural fraction may be suspended in solution as colloids as well as occurring as part of the stationary solids. 

Dissolution of minerals, such as may occur during dissimilatory Fe(lll) reduction, or precipitation of new biominerals during reductive or oxidative processing of Fe, represent important steps in which Fe isotope fractionation may occur. We briefly review several experiments that have investigated the isotopic effects during mineral dissolution, as well as calculated and measured isotopic fractionations among aqueous Fe species and in fluid-mineral systems. In some studies, the speciation of aqueous Fe is unknown, and we will simply denote such cases as Fe(lll)jq or Fe(ll)aq. 

Solubility products can be used to predict the stability of a mineral by comparing the observed ion product, [A (aq)][B (aq)], to the mineral s K. K the ion product is greater than K, the solution is supersaturated with respect to that mineral. In this case, precipitation should proceed spontaneously until the ion concentrations are decreased to levels that lower the ion product to the value dictated by the K. Conversely, if the ion product is less than the K, the solution is undersaturated with respect to that mineral. Dissolution should proceed spontaneously until the ion concentrations are increased to levels that raise the ion product to a value equal to the K. At equilibrium, where the ion product has a value equal to that of the K, the rate of mineral dissolution is equal to the rate of precipitation, so the ion concentrations remain constant over time. 

Armstrong attributed the increased resistance of dentin matrix to proteolysis to the blockage of susceptible sites by covalently bound carbohydrate. Later it became clear that the Maillard reaction induces the formation of covalent bonds (cross-links) between protein molecules, accounting for such resistance as well. The presence of non-degradable matrix proteins inhibits mineral dissolution (Chapter 2). In addition, both brown pigments and cross-linked proteins inhibit the production of extracellular polysaccharides by cariogenic streptococci (Kobayashi et al., 1990). 

Bone dissolution is composed of two major processes mineral dissolution and protein degradation. Bone mineral is hydroxyapatite i.e. (Ca3(P04)2)3 x Ca(OH)2. It is dissolved into Ca, HP04, and H2O with the help of hydrochloric acid. For this, independently protons (H" ) and chloride (Cl ) are... 

Moreover, as expected disruption of genes coding for enzymes critical to the function of osteoclast such as tartrate-resistant acid phosphatase (Hayman et al., 1996) and cathepsin K (Gowen et al., 1999) also produced osteopetrosis. This complements earlier discussed spontaneous osteopetrotic phenotypes produced by interception of pathways generating either protons or chloride necessary for mineral dissolution. 

Dissolution and precipitation in the subsurface are controlled by the properties of the solid phases, by the chemistry of infiltrating water, by the presence of a gas phase, and by environmental conditions (e.g., temperature, pressure, microbiological activity). Rainwater, for example, may affect mineral dissolution paths differently than groundwater, due to different solution chemistry. When water comes in contact with a solid surface, a simultaneous process of weathering and dissolution may occur under favorable conditions. Dissolution of a mineral continues until equilibrium concentrations are reached in the solution (between solid and liquid phases) or until all the minerals are consumed. 

The initial compositions of both the infiltrating water and the solid materials may change due to their interaction, which in turn may affect the solubility and the pathway of dissolution-precipitation processes with time. When a particular component of the dissolved solution reaches a concentration greater than its solubility, a precipitation process occurs. Table 2.1 includes the solubility of selected sedimentary minerals in pure water at 25°C and total pressure of 1 bar, as well as their dissolution reactions. All of the minerals listed in Table 2.1 dissolve, so that the products of the mineral dissolution reactions are dissolved species. Figure 2.2 shows the example of gypsum precipitation with its increasing concentration in a NaCl aqueous solution. 

The subsurface generally is an open system. The presence of CO and other gases in the atmosphere affects the partial pressure of gas constiments in the subsurface. For example, carbonate mineral dissolution in a system open to atmospheric COj does not achieve equilibrium. However, higher local subsurface CO concentrations can originate from biological activity and other oxidation processes. 

The rate of chemical weathering of minerals in the subsurface depends on a number of factors, including mineralogy, temperature, flow rate, surface area, presence of ligands and CO, and H+ concentrations in the subsurface water (Stumm et al. 1985). Figure 2.3 shows the rate-limiting steps in mineral dissolution consisting of... 

Fig. 2.3 Rate-limiting steps in mineral dissolution (a) transport-controlled, (b) surface reaction-controlled, and (c) mixed transport and surface reaction control. Concentration (C) versus distance (r) from a crystal surface for three rate-controUing processes, where is the saturation concentration and is the concentration in an infinitely diluted solution. Reprinted from Sparks DL (1988) Kinetics of soil chemical processes. Academic Press New York 210 pp. Copyright 2005 with permission of Elsevier...

Novak CF (1993) Modelhng mineral dissolution and precipitation in dual-porosity fracture-matrix system. J Contam Hydrol 13 91-115... 

Table 8.26 pH dependence of mineral dissolution rates (after Lasaga, 1984 with integrations). 

Table 1 Important weathering reactions in order of ease of chemical weathering and solubility, which goes along with the reaction rate of the mineral dissolution, except for bacterial mediated pyrite oxidation [9, 10]...

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