Feldspar diffusion

ChemiakDJ (2002) Ba diffusion in feldspar. Geochim Cosmochim Acta 66 1641-1650 Cherdyntsev W, Kishtsina Gl, Kuptsov VM, Kuzmina YA, Zverev VL (1967) Radioactivity and absolute age of yoimg volcanic rocks. Geokhimiya 7 755-762... 

Foland, K. A. (1974). Ar40 diffusion in homogeneous orthoclase and an interpretation of Ar diffusion in K-feldspars. Geochim. Cosmochim. Acta, 38, 151-66. 

Non-lattice sites may play an important role in the incorporation of large foreign ions in crystal structures during coprecipitation Pingitore (Chapter 27) discusses the importance of these sites in the formation of coprecipitates of calcium carbonate containing Srz+ or Ba. White and Yee (Chapter 28) discuss the diffusion of alkali ions into defect structures in the surfaces of glasses and crystalline feldspars. 

The preceding data, though limited in nature, represent one of the first attempts to measure solid state diffusion rates of alkali elements into the near-surface region of feldspars and natural glasses at low temperature. As such, interesting comparisons can be made with diffusion coefficients and activation energies calculated from numerous high temperature isotope and tracer diffusion studies f 11-181. 

At least four different explanations have been proposed to account for parabolic kinetics. The oldest and best established is the "protective-surface-layer" hypothesis. Correns and von Englehardt (6) proposed that diffusion of dissolved products through a surface layer which thickens with time explains the observed parabolic behavior. Garrels ( 12, 1 3) proposed that this protective surface consists of hydrogen feldspar, feldspar in which hydrogen had replaced alkali and alkaline earth cations. Wollast (j>) suggested that it consists of a secondary aluminous or alumino-silicate precipitate. In either case, a protective surface layer explains parabolic kinetics as follows If the concentration of any dissolved product at the boundary between the fresh feldspar... 

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). 

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

However, it must be emphasized that interpretation of elemental diffusion in feldspars is complicated by the structural state of the polymorphs, which vary in a complex fashion with temperature, chemistry and re-equilibration kinetics. These complexities also account for the controversies existing in the literature regarding diffusion energy in these phases (see also, incidentally, figure 4.8). Elemental dilfusivity data for rock-forming silicates are listed in table 4.8. 

Figure 4.8 Arrhenius plot of elemental diffusivity in feldspars. From Yund (1983). Reprinted with permission of The Mineralogical Society of America.

Giletti B. J., Sennet M. R, and Yund R. A. (1978). Studies in diffusion. III Oxygen in feldspars—an ion microprobe determination. Geochim. Cosmochim. Acta, 42 45-57. 

Lin T. H. and Yund R. A. (1972). Potassium and sodium self-diffusion in alkali feldspar. Contrib. Mineral Petrol, 34 177-184. 

Misra N. K. and Venkatasubramanian V. S. (1977). Strontium diffusion in feldspars—a laboratory study. Geochim. Cosmochim. Acta, 41 837-838. 

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