Feldspar weathering soils

Berner, R. A., and Holdren, G. R., Jr. (1979). Mechanism of feldspar weathering. II. Observations of feldspars from soils. Geochim. Cosmochim. Acta 43, 1173-1186. 

Feldspars weather directly to ionic components by hydrolysis and H attack however, secondary minerals may form as products of the weathering reaction, depending on the chemical environment in the soil. In a closed system where feldspar is not exposed to acidity generated within or outside the soil, hydrolysis is the dominant reaction, producing an alkaline solution ... 

Not much is known for certain about the rate of weathering of secondary minerals. Gibbsite dissolves in acid solutions (pH 2-3) at a rate per unit of surface area that is not greatly different from that of feldspars (A 10 " moles/mVsec). Naturally, because gibbsite crystals tend to be much smaller (micron-sized) than particles of feldspars and other primary minerals, actual dissolution rates of feldspars and gibbsite on a mass basis are very different. Consequently, when H" ions enter acid soils, they are more likely to react with clays and thereby dissolve Al , rather than be consumed by the process of primary mineral (e.g, feldspar) weathering. 

A second chemical factor affecting mineral weatherability is the position of ions in the structure. The tetrahedra of Ca feldspars contain half Al3+ and half Si4+. At room temperature, Al3+ is more stable in octahedral coordination, The charge deficit created by the Al3+ substitution is made up by Ca2+ ions between the tetrahedra. The structural strain, the charge deficit in the tetrahedra, and concentrated Ca2+ counter charge weaken the anorthite feldspar structure with respect to weathering relative to Na and K feldspars. In Na and K feldspars, only one-quarter of the tetrahedral positions are occupied by A1 and that charge deficit can be locally neutralized by Na+ or K+. Calcium feldspars are, therefore, the least stable feldspars under soil conditions. Potassium feldspars are more stable than Na feldspars, because K fits better between adjacent tetrahedra. 

Occasionally the course of weathering can reverse in the sense that feldspar weathering will create secondary silicates that are unstable under the leaching and weathering conditions of well drained soils. Such reversal occurs because of the accumulation of K+, Na+, Ca2+. Mg2, and Si(OH)4 in arid and poorly drained soil solutions. These secondary silicates include zeolites, evaporites, and the authigenic feldspars. The area denoted as soil solution in Fig. 7.6 shows the extreme concentrations that have been reported in soil solutions. Within this range, several silicate minerals are stable. 

Berner RA, Holdren GR Jr (1979) Mechanism of feldspar weathering. II. Observations of feldspars from soils. Geochim Cosmochim Acta 43 1173-1186 Bevan J, Savage D (1989) The effect of organic acids on the dissolution of K-feldspar under conditions relevant to burial diagenesis. Mineral Mag 53 415-425 Blum AE, Lasaga AC (1988) The role of surface speciation in the low-temperature dissolution of minerals. Nature 4 431-433... 

An extensive chemical and microscopic study of mica separated from 21 Piedmont soils from the Piedmont of southeastern United States was made by Denison et al. [1929]. The fraction analyzed was greater in size than that passed through a 200-mesh sieve. Weathering was evident in even these sand-size particles. In all soil profiles, biotite seemed to be altered to the same extent, the K2O content usually approximated 4%. On the other hand, muscovite, as described by the authors, had a wide variance in K2O content. This muscovite in some profiles contained less than 1 % K2O, whereas in other profiles, the mineral contained as much as 9% K2O. The authors proposed that there were two kinds of muscovite present—some secondary, having formed from the products of potassium-feldspar weathering, and some primary, which muscovite inherited from the parent material. The identification of material with 1 % K2O as muscovite would be questioned today. The use of X-ray diffraction for soil clay identification occurred after this microscopic work was done. It would have been useful to examine the < 200-mesh particles to supplement the work done with the microscope on particles of a size greater than 200 mesh. 

The reporting of mica in soil clays depends somewhat on the method of detection. Jackson and Mackenzie [1964] state that some soil clays, which show no indication of mica based on X-ray diffraction, may contain from 5 to 20 % or more of micas based on chemical analysis and on the basis of 10% K2O in mica. According to Schuffelen and van der Marel [1955], soils high in allophane fix very considerable quantitities of potassium. Thus, potassium does not necessarily reside altogether in micas and feldspars in soils. Some of it may be in amorphous material. However, some of the potassium may be in micalike zones of particles, which are largely montmorillonite or vermiculite and have weathered from micas. Such zones may be too small to be detected by X-ray diffraction (Knibbe and Thomas [1972]). 

Gays are an essential component of soils, to which we owe our survival, and they are also the raw materials for some of mankind s most ancient and essential artefacts pottery, bricks, tiles, etc. Clays are formed by the weathering and decomposition of igneous rocks and occur typically as very fine panicles e.g. kaolinite is formed as hexagonal plates of edge. 1-3 p m by the weathering of alkaline feldspar... 

In the simulation, CO2 in the soil gas reacts with the feldspars, leading to the alkali leaching and separation of silica from alumina observed to result from soil weathering. Near the top of the profile, the reaction produces gibbsite and adds Na+, K+, and Si02(aq) to the migrating pore fluid, according to the reactions,... 

Wilson (28) noted the presence of etch pits (crystallographically controlled voids or features of negative relief, or "negative crystals") on some soil feldspars, and reviewed similar observations from earlier studies. Some examples of etch pits on naturally weathered feldspars are shown in Figure 3- Etch pits... 

The feldspars are aluminosilicates in which up to half the Si44 ions have been replaced by Al3+ ions. They are the most abundant silicate materials on Earth and are a major component of granite, a compressed mixture of mica, quartz, and feldspar (Fig. 14.45). When some of the cations between the crystal layers are washed away as these rocks weather, the structure crumbles to clay, one of the main inorganic components of soil. A typical feldspar has the formula KAlSi3Og. Its weathering by carbon dioxide and water can be described by the equation... 

Clay is formed from the weathering of igneous rocks containing the mineral, feldspar. Clay occurs abundantly in nature and is found in all soils. Only its percentage differs. Clay contains fine silica, alumina and iron oxide. 

The application of chemical kinetics to weathering processes of soil minerals first appeared in the work of Wollast (1967). He concluded that the rate-limiting step for weathering of feldspars was diffusion (Chapter 7). This work touched off a lively debate that is still raging today about whether weathering of feldspars and ferromagnesian minerals is controlled by chemical reaction (CR) or diffusion. 

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