Mineral weathering field studies

So far we have discussed weathering rates and rate laws from laboratory experiments on pure minerals. These laboratory studies are meant to provide insight for natural systems (rates and variables that affect these rates). We may first try to compare laboratory and field results. 

The rates of chemical weathering reactions increase with temperature as aU chemical processes. This effect has been confirmed in laboratory experiments and field studies involving silicate rocks [12]. Weathering reactions occur at mineral... 

Several simple experimental systems that simulate some aspect of the groundwater environment have been used to study the breakdown of individual minerals. These kinetics studies have encompassed quartz (Brantley et al., 1986), feldspars (Holdren and Berner, 1979 Holdren and Speyer, 1985), pyroxenes and amphiboles (Berner and Schott, 1982 Schott and Berner, 1985), carbonates (Berner, 1978), and glasses (White, 1983). The relative stability observed in laboratory weathering is consistent with field-based observations however, experimental rates appear to be faster than those in natural systems. 

Many minerals have been found to dissolve and precipitate in nature at dramatically different rates than they do in laboratory experiments. As first pointed out by Paces (1983) and confirmed by subsequent studies, for example, albite weathers in the field much more slowly than predicted on the basis of reaction rates measured in the laboratory. The discrepancy can be as large as four orders of magnitude (Brantley, 1992, and references therein). As we calculate in Chapter 26, furthermore, the measured reaction kinetics of quartz (SiC>2) suggest that water should quickly reach equilibrium with this mineral, even at low temperatures. Equilibrium between groundwater and quartz, however, is seldom observed, even in aquifers composed largely of quartz sand. 

Mineral composition and structure are the primary intrinsic factors controlling weathering rates. Based on early weathering studies, Goldich (1938) observed that the weathering sequence for common igneous rocks in the field was the reverse of Bowen s reaction series that ranked minerals in the order of crystalhzation from magma. Amphi-boles and pyroxenes are expected to weather faster than feldspars which weather faster than... 

Kinetics of chemical weathering are important to understand the rates of rock dissolution, sediment formation, and acid neutralization in the environment. In this chapter, there are three principal objectives (1) to describe the theoretical kinetics of chemical weathering for pure minerals, (2) to discuss the relative advantages and disadvantages of various experimental apparatus for measuring those kinetics in soils, and (3) to make comparisons between laboratory and field measurements of weathering rates and solute transport. A case study of laboratory and field measurements at Bear Brook Watershed, east of Orono, Maine, at Lead Mountain, will be used to illustrate the principles discussed in this chapter. 

Blomquist J, Kjall P, Malmstrom M, Banwart S (1996) Mossbauer studies of the weathering process in some mica minerals. Conf Proc, IntT Conf Appl Mossbauer Effect 1995, SIF, Bologna, 721-724 Bonnin D, Muller S (1981) Study of the electric-field gradient in muscovite by the Mossbauer spectrometry of iron. Phys Stat Solidi C-Basic Res 105 647-657... 

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