Mineral precipitation rate

Lasaga, A. C. (1995). Fundamental approaches in describing mineral dissolution and precipitation rates. In Chemical Weathering Rates of Silicafe Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America, Washington, DC, Reviews in Mineralogy 31, 23-86. 

The above-mentioned consideration indicates that important factors controlling the precipitations of barite and silica are surface area/water mass ratio (A/M), temperature, precipitation rate constant (k) and flow rate (u), and the coupled fluid flow-precipitation models are applicable to understanding the distributions of minerals in submarine hydrothermal ore deposits. 

The calculations based on four reservoir models were made using equations (1-62)-(l-67) and precipitation rate constant k) for Si02 minerals by Rimstidt and Barnes (1980). 

Assuming the ranges of A/M, flow rate of mixed fluid, porosity and giving the precipitation rate of Si02 minerals (Rimstidt and Barnes, 1980), the relationship between dissolved silica concentration of mixed fluid and temperature was obtained (Fig. 1.142). It was found that the porosity does not change the results of calculations. 

Mass transfer can be described in more sophisticated ways. By taking in the previous example to represent time, the rate at which feldspar dissolves and product minerals precipitate can be set using kinetic rate laws, as discussed in Chapter 16. The model calculates the actual rates of mass transfer at each step of the reaction progress from the rate constants, as measured in laboratory experiments, and the fluid s degree of undersaturation or supersaturation. 

Do the kinetic rate constants and rate laws apply well to the system being studied Using kinetic rate laws to describe the dissolution and precipitation rates of minerals adds an element of realism to a geochemical model but can be a source of substantial error. Much of the difficulty arises because a measured rate constant reflects the dominant reaction mechanism in the experiment from which the constant was derived, even though an entirely different mechanism may dominate the reaction in nature (see Chapter 16). 

In this chapter we consider how to construct reactions paths that account for the effects of simple reactants, a name given to reactants that are added to or removed from a system at constant rates. We take on other types of mass transfer in later chapters. Chapter 14 treats the mass transfer implicit in setting a species activity or gas fugacity over a reaction path. In Chapter 16 we develop reaction models in which the rates of mineral precipitation and dissolution are governed by kinetic rate laws. 

We might, for example, study the rate at which the ferrous ion Fe++ oxidizes by reaction with O2 to produce the ferric species Fe+++. Since the reaction occurs within a single phase, it is termed homogeneous. Reactions involving more than one phase (including the reactions by which minerals precipitate and dissolve and those involving a catalyst) are called heterogeneous. 

Despite the authority apparent in its name, no single rate law describes how quickly a mineral precipitates or dissolves. The mass action equation, which describes the equilibrium point of a mineral s dissolution reaction, is independent of reaction mechanism. A rate law, on the other hand, reflects our idea of how a reaction proceeds on a molecular scale. Rate laws, in fact, quantify the slowest or rate-limiting step in a hypothesized reaction mechanism. 

The dissolution rate, according to the theory, does not depend on the mineral s saturation state. The precipitation rate, on the other hand, varies strongly with saturation, exceeding the dissolution rate only when the mineral is supersaturated. At the point of equilibrium, the dissolution rate matches the rate of precipitation so that the net rate of reaction is zero. There is, therefore, a strong conceptual link between the kinetic and thermodynamic interpretations equilibrium is the state in which the forward and reverse rates of a reaction balance. 

Comparing the development here to the accounting for the kinetics of mineral precipitation and dissolution presented in the previous chapter (Chapter 16), we see the mass transfer coefficients v and so on serve a function parallel to the coefficients v , etc., in Reaction 16.1. The rates of change in the mole number of each basis entry, accounting for the effect of each kinetic redox reaction carried in the simulation, for example,... 

In calculating most of the reaction paths in this book, we have measured reaction progress with respect to the dimensionless variable . We showed in Chapter 16, however, that by incorporating kinetic rate laws into a reaction model, we can trace reaction paths describing mineral precipitation and dissolution using time as the reaction coordinate. 

The absence of secondary mineral precipitation in fungal tunnels indicates that the weathering products are removed from the tunnel interior, either by diffusion or fungal transport. Removal by diffusion is likely because it is probably faster than feldspar weathering. If the tunnel is occupied by a fungus, the diffusion rate would be depressed, so that fungal transport could become important. 

Lichtner (2001) developed the computer code FLOTRAN, with coupled thermal-hydrologic-chemical (THC) processes in variably saturated, nonisothermal, porous media in 1, 2, or 3 spatial dimensions. Chemical reactions included in FLOTRAN consist of homogeneous gaseous reactions, mineral precipitation/dissolution, ion exchange, and adsorption. Kinetic rate laws and redox... 

Empirical models predicting the rates of mineral-specific dissolution as a function of pH are summarized within the section on mineral composition in an attempt to provide a useful database for predicting dissolution rates for both laboratory and field systems. Equations describing near-equilibrium mineral dissolution and precipitation rates are summarized in the section on chemical affinity. 

S. S., and Balabin A. (1997) Change in the dissolution rates of alkali feldspars as a result of secondary mineral precipitation and approach to equilibrium. Geochim. Cosmochim. Acta 59, 19-31. 

The calculation of rates based on changes in solute species concentrations in soils, aquifers, and watersheds requires partitioning the reactant between sources produced by primary mineral dissolution and sinks created by secondary mineral precipitation. Calculation of weathering rates based on solute transport requires knowing the nature and rate of fluid flow through soils, aquifers, and watersheds. 

Borensen C, Kirchner U, Scheer V, Vogt R, Zellner R (2000) Mechanism and kinetics of the reactions of NO2 or HNO3 with alumina as a mineral dust model compound. J Phys Chem A 104 5036-5045 Borys RD, Lowenthal DH, Mitchell DL (2000) The relationships among cloud microphysics, chemistry, and precipitation rate in cold mountain clouds. Atmos Environ 34 2593-2602 Bowles RK, McGraw R, Schaaf P, Senger B, Voegel JC, Reiss H. (2000) A molecular based derivation of the nucleation theorem. J Chem Phys 113 4524-4532... 

The rate constants for dissolution, kd,x. and precipitation, Jcpx (mol m s ) refer only to the species X. If the mineral dissolves congruently, that is, if the ratio of the constituents released to solution is the same as in the solid, then the solid stoichiometric coefficient times the total mineral dissolution and precipitation rate constants can be used. 

In recent years rapid advances have been made in understanding mechanisms and controls on the dissolution and precipitation rates of silicate and aluminosilicate minerals in general. Much of this work has focused on rates of chemical weathering (cf. Steefel and Van Cappellen 1990 Lasaga... 

There are other possible explanations when a model calculation indicates a water is supersaturated with respect to one or more carbonate minerals. They include (1) the use of inaccurate, inconsistent, or incomplete thermodynamic data for carbonate minerals and aqueous complexes (2) nonstoichiometry (i.e.. solid solution) and/or small (submicron) particle sizes of the carbonates, making them more soluble than the well-crystallized pure phases assumed in the calculation (cf. Busenberg and Plummer 1989) (3) different solution models used to define the mineral and in the calculation of saturation state in a natural water (4) inhibition of carbonate nucieation by adsorbed substances (cf. Inskeep and Bloom 1986) and (5) slow nucieation and precipitation rates that require times exceeding residence times of the water in the water-rock system (cf. Herman and Lorah 1987). The same possible explanations apply to model-computed supersaturations obtained for noncarbonate minerals. 

Know the general conditions of occurrence and stability of the major clay mineral groups, including the roles played by parent mineralogy, temperature, precipitation rate, soil drainage, and soil maturity in clay occurrence. 

It is interesting to note the different concentration profiles exhibited by silica and aluminium, because these provide indirect evidence of the possible dissolution and/or precipitation rates of different aluminosilicate minerals in the reservoir when CO2 is added to the system. Silica exhibits the same... 

For equilibrium to be reached, all elementary processes must have equal forward and backward rates. This differs from steady-state conditions, where only certain reactions and processes have balanced rates. Thus, at equilibrium, dissolution and precipitation rates of minerals should become necessarily equal. Note also that under equilibrium conditions the net flux of dissolved components at the water-mineral interface is equal to zero and the eventual limitation of the rate by the transport of reactants and products disappear. Close to equilibrium, the reaction kinetics always become entirely controlled by the surface reactions. 

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