Chemical weathering plagioclase

Clayton, J. L. (1986). An estimate of plagioclase weathering rate in the Idaho batholith based upon geochemical transport rates. In "Rates of Chemical Weathering of Rocks and Minerals" (S. M. Coleman and D. P. Dethier, eds). Chap. 19, pp. 453-466. Academic Press, New York. 

These alkaline conditions are due in part to the consumption of acid during the low-temperature chemical weathering of plagioclase in a process similar to Eq. 19.4. 

X 10"4 equivalents per liter and corresponds to the destruction of about 3.5 X 10 4 moles of plagioclase, 0.2 X 10"4 moles of biotite, and about 0.2 X 10 4 moles of K-spar. The annual precipitation in this part of the Sierra averages about 100 cm./year. Therefore, the rate of chemical weathering is about 3.6 X 10"r moles/year/sq. cm. If the rock consists of one-third plagioclase by volume, the rock should be disintegrated to an average depth of one meter in about 9000 years, and the residue would be chiefly a rubble of quartz, K-feldspar, and kaolinite. 

Breakdown of plagioclase to montmorUlonite. The breakdown of plagioclase crystals or glass fragments in volcanic ash during chemical weathering may be described by the reaction ... 

Figure 3. SEM/EDX images of (a) ferric hydroxide coating on a feldspar grain, (b) EDX spectra of ferric hydroxide on a quartz grain, (c) chemical weathering of biotite platelets, and (d) etch pits on a plagioclase mineral grain. All grains were taken from surface sediments of Lake Cristallina, Switzerland. Photographs are courtesy of Professor Rudolf Giovanoli, Laboratory of Electron Microscopy, University of Bern.

The weathering rate of insoluble silicate minerals is very low. Field balance studies showed that their actual values are by several orders of the magnitude (up to 5) lower than those observed in lab experiments. This may be due to a thick diffusion layer and rather high content of dissolved orthosilldc add. Nevertheless, as observations show, at weathering first to disappear are plagioclases, biotite and hornblendes, whereas quartz and orthoclase remain. Sequential disappearance of different silicate minerals (Table 2.25) shows their difference in the rate of chemical weathering. 

Goldich (1938) examined the mineral assemblages present in soil (Appendix, Plate 5) under a variety of environmental conditions and established a stability series for sand and silt-sized particles that illustrates the relative stability of primary silicate minerals (Goldich s weathering series) (Fig. 1.8). For example, Ca-plagioclase, olivine (Appendix, Plate 6) and pyroxene (Appendix, Plate 7) tend to be most easily suffered chemical weathering and quartz and mica are most resistant to the weathering. This order is quite consistent with calculated solubility (Fig. 1.7) and experimentally determined dissolution rate of silicate minerals (Fig. 1.10). The solubility and dissolution rate of silicate minerals are related to the crystal structures, which is described below. 

The continental pattern for Na matches the pattern for total feldspar percentages, as Na values are primarily correlated with plagioclase (Eberl Smith 2009). Feldspars are much more susceptible to chemical dissolution than quartz and, with sufficient time and precipitation, will weather mainly to clay minerals. As a result, total feldspar contents and Na contents decrease with increasing precipitation from west to east (Fig. 3). 

In addition to the hydrothermal controls, the elemental cycles of Ca and Mg are also linked to those of CO2 and Si02 because the mobilization of Ca and Mg during terrestrial weathering from silicates, such as plagioclase, involves consumption of CO2. In the case of calcium, the net chemical reaction is... 

This is a very sketchy depiction of the deep carbon cycle because it illustrates only the behaviors of calcium and silica. In reality, a wide variety of other cations are present in the silicate minerals, such as in the plagioclase feldspars (Table 13.2). Furthermore, not all of the limestone is converted into siUcate minerals some remains as limestone. Uplift of the limestone onto land, followed by chemical and biological weathering, is another sink for atmospheric CO2, via... 

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