Feldspars chemistry

Table 5.62 lists the chemical compositions of natural feldspars, taken from Deer et al. (1983). Feldspar chemistry is solved on a 32-oxygen anhydrous basis, corresponding to four formula units. The X cation summation is quite close to the stoichiometric value (4) the Z cations are also close to the theoretical value (16). 

Of the approximately 5 million metric tons of feldspar prodncts produced annually, Italy is the leading supplier with about 29%, followed by the US with about 12%. Other major producers are Japan (aplite), France, Germany, Korea, and Thailand. The two major applications for feldspar worldwide are glass and ceramics, which rely more on feldspar chemistry than mineralogy. In the US, 54% is used in glass and 44% in ceramics. 

Once the initial equilibrium state of the system is known, the model can trace a reaction path. The reaction path is the course followed by the equilibrium system as it responds to changes in composition and temperature (Fig. 2.1). The measure of reaction progress is the variable , which varies from zero to one from the beginning to end of the path. The simplest way to specify mass transfer in a reaction model (Chapter 13) is to set the mass of a reactant to be added or removed over the course of the path. In other words, the reaction rate is expressed in reactant mass per unit . To model the dissolution of feldspar into a stream water, for example, the modeler would specify a mass of feldspar sufficient to saturate the water. At the point of saturation, the water is in equilibrium with the feldspar and no further reaction will occur. The results of the calculation are the fluid chemistry and masses of precipitated minerals at each point from zero to one, as indexed by . 

O Neil JR, Taylor Jr HP (1967) The oxygen isotope and cation exchange chemistry of feldspar. J Geophys Res 74 6012-6022... 

J. (1955) The hydrothermal chemistry of silicates. Part VI. A lamellar habit in synthetic feldspar./. Chem. Soc., 2480-2481. 

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 5.55 Mutual dependence of Q i and Q d order parameters. In the upper part of the figure is outlined the T dependence of substitutional disorder Qod for different values of Qdi and, in the lower part, the T dependence of the displacive disorder parameter Qdt for different values of The heavy lines on the surface of local curves represent the solution for thermal equilibrium. From E. Salje and B. Kuscholke, Thermodynamics of sodium feldspar II experimental results and numerical calculations. Physics and Chemistry of Minerals, 12, 99-107, figures 5-8, copyright 1985 by Springer Verlag. Reprinted with the permission of Springer-Verlag GmbH Co. KG.

The activities of solute species in an aqueous solution in equilibrium with K-feldspar at the P and T of interest will be those dictated by equation 8.232. Let us now imagine altering the chemistry of the aqueous solution in such a way that the activities of the aqueous species of interest differ from equilibrium activities. New activity product Q ... 

Ribbe P. H. (1983a). The chemistry, structure and nomenclature of feldspars. In Reviews in Mineralogy, vol. 2 (2d ed.), P. H. Ribbe (series ed.), Mineralogical Society of America. 

A wide variety of zeolites are known to form in saline lakes where the species present is dependent upon the chemistry of the solutions. Rapid zeolite formation is aided by the existence of the volcanic glass and high water salinities. Potassium feldspar occurs with the common alkali zeolites (Hay and Moiola, 1963 Hay, 1964 Hay, 1966 Sheppard and Gude, 1969, 1971), however, albite is not evident as a diagenetic mineral in saline lakes. 

Benes P, Borovec Z, Strejc P. 1986. Interaction of radium with freshwater sediments and their mineral components III. Muscovite and feldspar. Journal of Radioanalytical and Nuclear Chemistry 98 91-103. 

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. 

Aluminum occurs widely in nature in silicates such as micas and feldspars, complexed with sodium and fluorine as cryolite, and in bauxite rock, which is composed of hydrous aluminum oxides, aluminum hydroxides, and impurities such as free silica (Cotton and Wilkinson 1988). Because of its reactivity, aluminum is not found as a free metal in nature (Bodek et al. 1988). Aluminum exhibits only one oxidation state (+3) in its compounds and its behavior in the environment is strongly influenced by its coordination chemistry. Aluminum partitions between solid and liquid phases by reacting and complexing with water molecules and anions such as chloride, fluoride, sulfate, nitrate, phosphate, and negatively charged functional groups on humic materials and clay. 

Aluminosilicates form an extensive family of compounds that include layered compounds (such as clays, talc, and micas), 3-D compounds, (e.g. feldspars, such as granite), and microporous solids known as molecular sieves. The structural diversity of these materials is contributed to by aluminum s ability to occupy both tetrahedral and octahedral holes as it also does in y-Al203. Thus, aluminum substitution for silicon in silicate minerals may lead to replacement of silicon in tetrahedral sites or the aluminum can occupy an octahedral environment external to the silicate lattice. Replacement of Si with Al requires the presence of an additional cation such as H+, Na+, or 0.5 Ca + to balance the charge. These additional cations have a profound effect on the properties of the aluminosilicates. This accounts for the many types of layered and 3-D structures (see Silicon Inorganic Chemistry). 

Aluminum is a constituent of many minerals, including clay (ka-olinite), mica, feldspar, sillimanite, and the zeolites. Some of these minerals are discussed under the chemistry of silicon, in Chapter 31. Aluminum oxide (alumina), occurs in nature as the mineral corundum. Corundum is the hardest of aU naturally occurring substances except diamond it scratches all other minerals, but is itself scratched by diamond, and also by the artificial substances boron carbide, and silicon carbide, SiC. Corundum and impure corundum (emery) are used as abrasives. 

Papike J. J. (1998) Comparative planetary mineralogy chemistry of melt-derived pyroxene, feldspar, and olivine. In Planetary Materials, Reviews in Mineralogy (ed. J. J. Papike). Mineralogical Society of America, Washington, DC, vol. 36, chap. 7, pp. 7-10-7-11. 

Development of alteration layers on minerals dissolved under neutral and alkaline conditions has not been thoroughly investigated, but some work has been completed, especially on feldspar compositions (Chou and Wollast, 1984 Hellmann et al, 1989, 1990a Muir et al, 1990 Nesbitt et al, 1991 Hellmann, 1995, Hamilton et al, 2000). Under neutral conditions, the leached layer thickness (tens of angstroms to a few hundred angstroms) is generally less than that observed for more acid dissolution, with variability reported in the composition and thickness of the layer i.e., sodium depletion is generally observed, but both aluminum depletion and enrichment (with respect to silicon) have been reported. Variations in solution chemistry (see Section 5.03.7) and feldspar composition may explain some of these differences for... 

The alkali feldspar model of Equation (21) (Oelkers et al., 1994) differs from the anorthite model of Equation (24) (Oelkers and Schott, 1995b) in the assumed chemistry of the precursor for alkali feldspars, the precursor contains no aluminum (and thus dissolution depends on aluminum activity in solution) while for anorthite, the precursor contains aluminum (and therefore exhibits no dependence on aluminum activity in solution). In the first model, Al-H exchange occurs at the mineral surface, while in the second model H adsorption is the precursor to dissolution. These models have also been compared to glass dissolution for compositions ranging from albite to nepheline (Hamilton et al., 2001). 

Casey W. H., Westrich H. R., and Arnold G. W. (1988b) Surface chemistry of labradorite feldspar reacted with aqueous solutions at pH = 2, 3 and 12. Geochim. Cosmochim. Acta 52, 2795-2807. 

Gout R., Oelkers E. H., Schott J., and Zwick A. (1997) The surface chemistry and structure of acid-leached albite new insights on the dissolution mechanisms of the alkali feldspars. Geochim. Cosmochim. Acta 61(14), 3013-3018. 

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