Stoichiometric dissolution

Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. 

One additional aspect of laboratory dissolution experiments is the question of stoichiometric vs. non-stoichiometric dissolution. Many of the studies cited above analyzed only a few of the elements released by feldspar that is, although alkalis, alkaline earths, silica, and aluminum may be released during dissolution of feldspar, few studies report analyses for all elements. Often, only silica was analyzed. Where multiple elements are analyzed, they are often released to the solution in proportions which do not correspond to the bulk stoichiometry of the feldspar ( ] ). 

Usually, alkalis and alkaline earths are released in excess of silica, and dissolved aluminum is the least abundant. This observed non-stoichiometry suggested that silica and aluminum were being preferentially retained in some solid phase relative to alkalis and alkaline earths. Such preferential retention, manifested as non-stoichiometric dissolution, was long thought to be consistent with the concept of some kind of residual surface layer. 

The presence of the metal cation and an appropriate anion (OH-, Cl-, F-, etc.) in contact with the surface of the apatite is sufficient to cause partial/complete dissolution of the apatite and precipitation of the more stable pyromorphite. A typical set of stoichiometric dissolution and precipitation reactions would be (Lower et al. 1998b) ... 

Chou and Wollast (1984, 1985) employed a fluidized-bed reactor to study albite dissolution with time. Figure 7.3 shows a short-term experiment run at room temperature and pressure using water as the input solution. There is a fast nonstoichiometric dissolution early in the reaction period that decreases rapidly until a steady state is approached. Linear kinetics and stoichiometric dissolution prevail later. If the pH of the input solution is changed, however, there is an increase in dissolution rate (Fig. 7.4) similar to the beginning of an experiment (Fig. 7.3). 

Steps 1, 2, and 3 account for the initially high rate of dissolution of alkali and alkaline earth metals (Na", K, Ca ) from feldspars, and the relative lack of silica and aluminum dissolution. This is termed incongruent (nonstoichiometric) dissolution, meaning that a portion of the mineral structure is dissolving selectively, leaving a residue enriched in silica and alumina. Step 4 accounts for the later stage of congruent (stoichiometric) dissolution, in which the elements are released into solution in proportion to their mole fractions in the structure. 

Calcareous minerals and evaporite minerals (haUdes, gypsum) are very soluble and dissolve rapidly and, in general, congmendy, ie, yielding upon dissolution the same stoichiometric proportions in the solution as the proportions in the dissolving mineral and without forming new soHd phases (Fig. 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

At first glance, the HRC scheme appears simple the polymer is activated, dissolved, and then submitted to derivatization. hi a few cases, polymer activation and dissolution is achieved in a single step. This simplicity, however, is deceptive as can be deduced from the following experimental observations In many cases, provided that the ratio of derivatizing agent/AGU employed is stoichiometric, the targeted DS is not achieved the reaction conditions required (especially reaction temperature and time) depend on the structural characteristics of cellulose, especially its DP, purity (in terms of a-cellulose content), and Ic. Therefore, it is relevant to discuss the above-mentioned steps separately in order to understand their relative importance to ester formation, as well as the reasons for dependence of reaction conditions on cellulose structural features. 

The sulfide halides TISX are prepared by heating a stoichiometric mixture of the thallium halogenide and sulfur in a sealed ampoule at 180°C for 30 h. The mixture is then slowly cooled to room temperature. The compounds TlSeX are obtained by reaction between thallium metal and selenide halide at 280°C during 40 h (22). On heating TlYCl to 500°C in vacuo, the compounds TI4YCI4 result (322). Dissolution of... 

As was mentioned in the introduction to this chapter "diffusion-controlled dissolution" may occur because a thin layer either in the liquid film surrounding the mineral or on the surface of the solid phase (that is depleted in certain cations) limits transport as a consequence of this, the dissolution reaction becomes incongruent (i.e., the constituents released are characterized by stoichiometric relations different from those of the mineral. The objective of this section is to illustrate briefly, that even if the dissolution reaction of a mineral is initially incongruent, it is often a surface reaction which will eventually control the overall dissolution rate of this mineral. This has been shown by Chou and Wollast (1984). On the basis of these arguments we may conclude that in natural environments, the steady-state surface-controlled dissolution step is the main process controlling the weathering of most oxides and silicates. 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

The term congruent dissolution often refers to a process by which a mineral is dissolved in stoichiometric proportions into solution without formation of a new solid phase, and this usage is convenient for isotopic studies because it constrains the phases or components that may... 

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