Plagioclase weathering soils

Bakker, M. R., George, E., Turpault, M.-P., Zhang, J. L., and Zeller, B. (2004). Impact of Douglas-fir and Scots pine seedlings on plagioclase weathering under acidic conditions. Plant Soil 266, 247-259. 

The over-all picture of what happens to the soil waters, as illustrated by Tables II and IV, is that initially they rapidly attack the rocks, kaolinizing chiefly plagioclase plus biotite and K-spar. As they penetrate more deeply, the reaction rate slows down, and both kaolinite and mont-morillonite are weathering products. Also, an important part of the Ca2+ comes from solution of small amounts of carbonate minerals. 

Plagioclase A sodium and calcium aluminum silicate mineral that commonly forms in igneous and metamorphic rocks, and occasionally survives weathering to occur in sediments, soils, and sedimentary rocks. A solid solution exists in plagioclase, where (CaAl)5+ and (NaSi)5+ may substitute for each other to produce a composition that ranges from NaAISisOg to CaAUSiiOg. 

Nesbitt H. W. and Muir I. J. (1988) SIMS depth profiles of weathered plagioclase and processes affecting dissolved A1 and Si in some acidic soils. Nature 334(6180), 336-338. 

Are there Earth-surface conditions for which the above minerals are thermodynamically stable and so would not weather The answer is rarely for the olivine minerals (cf. Bath et al. 1987), which rapidly disappear in the volcanic soils and beach sands of Hawaii, for example. What about the stability of the plagioclase feldspar Gibbs free energies of the reactants and products (in kcal/mol) given under weathering reaction (7.3) indicate that AG° = -23.53 kcal/mol, and = 1017.25 If we assume a typical soil pH of 6, then we can compute that Ca- would have... 

Of total silica in the rock, 22% has been lost to the soil solution, chiefly from weathering of the plagioclase feldspar, biotite, and hornblende. All or most of the quartz has evidently been unaffected by weathering. 

There is no proof, liowever, that the weathered material has derived from the exact parent rock given in the table. This may explain the excess in soil vol % quartz, which, assuming quartz is not dissolved or precipitated during soil formation and that volume and weight percents are roughly equal, should not exceed about (30/75) x 100 = 40 wt % of the soil. (Densities of quartz, K-feldspar, plagioclase feldspar, and kaolinite are alt 2.60 0.05 g/cm ). 

The results of the HSB are not presented here, as it is still too early to observe changes in soils after 4 years. Therefore, only the results from the TMB are reported here. It is also worth noting that whatever the field site and treatment in the TMB experiment, apatite buried for 4 years weathered much more rapidly than plagioclases that were buried for 3years, and the total mass dissolved from apatite and plagioclases was 0.8-2% and 0.15-0.2%, respectively. Because of the slow weathering rate of plagioclases, we present only the results for apatite. 

A comparison of the three soil components shows that the rhizosphere tends to be depleted compared to the bulk soil for many minerals and at most sites. This trend is particularly well expressed for chlorite and amphiboles (Table 1). Vermiculite and plagioclases follow the same general trend when the rhizosphere is considered as a whole, although the trend is inverted at one site for each mineral (Table 1). Micas show no specific trend. On the other hand, K-feldspars rather tend to be less weathered in the inner rhizosphere. 

Comparable results for plagioclases and K-feldspars were obtained by Courchesne and Gobran (1997) and April and Keller (1990). Both studies have been conducted in the field. Their results indicate that easily weathered minerals such as amphiboles and expandable phyllosilicates were depleted in the rhizosphere as compared to the bulk soil (Courchesne and Gobran, 1997). April and Keller (1990) also showed that the rhizoplan and the rhizosphere were depleted in biotite, a K-bearing mineral. Similarly, Kodama et al. (1994) and Hinsinger et al. (1993) conducted pot experiments that indicated a relative enrichment in vermiculite in the rhizosphere. 

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. 

Effect of pH. Most minerals, including carbonate minerals (Wollast 1990), exhibit a transition from pH-independent to pH-dependent dissolution (Fig. 5). The critical question is whether the pH of the transition is above or below the pH of the soil solution. The transition points for a number of minerals are shown in Table 2. If alkali feldspars and intermediate plagioclases are the most important sources of cations in weathering (Garrels and Mackenzie 1967), then the transition pH will be about 4.5, and the weathering rate will increase as What is a reasonable pH range for... 

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