Feldspar, Amphibole, and Pyroxene Dissolution Kinetics

Much attention has been given to feldspar dissolution kinetics over the past 20 years or so. This is largely attributable to feldspars being the most abundant minerals in igneous and metamorphic rocks. Their ubiquity in soils is also well known where they affect the resultant clay mineralogy and potassium status of soils. Another reason feldspar dissolution rates have been studied profusely has been the controversy over dissolution mecha- 

A number of workers (Wollast, 1967 Huang and Kiang, 1972 Luce et al., 1972) have observed that feldspar weathering conforms to the parabolic diffusion law (Chapter 2). An example of this is shown in Fig. 7.2. Much research effort has gone into explaining why parabolic kinetics could be operational for feldspar dissolution. Several explanations have been given, and these are discussed below. 

The existence or nonexistence of a residual layer has been studied using surface chemistry techniques such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) and solution chemistry calculations. Nickel (1973) calculated the thickness of a residual layer on albite from the mass of dissolved alkalis and alkaline earths released during laboratory weathering. The surface area was also measured, and the thickness of the residual layer was found to range from 0.8 to 8 nm. These results suggested a very thin layer, which would not cause parabolic kinetics. 

Chou and Wollast (1984, 1985) concluded that the behavior in dissolution rate when pH was changed was due to the formation of a new surface 

It is important to note that the layer thicknesses reported above were based strictly on solution chemistry analyses. Several reports have appeared on the thicknesses of leached layers using surface chemistry techniques. Petrovic et al. (1976) used XPS and analyzed K, Al, and Si content of altered K-feldspar grains and found the leached layer was 1.7 nm. Layer thicknesses for dissolution of enstatite, diopside, and tremolite based on XPS data are reported in Table 7.3. 

---

Rate-Limiting Steps in Mineral Dissolution 146 Feldspar, Amphibole, and Pyroxene Dissolution Kinetics 148 Parabolic Kinetics 149 Dissolution Mechanism 155 Dissolution Rates of Oxides and Hydroxides 156 Supplementary Reading 161... 

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. 

---