Reaction Kinetics of Primary Rock-forming Minerals under Ambient Conditions

Mineral dissolution kinetics influence such phenomena as development of soil fertility, amelioration of the effects of acid rain, formation of karst, acid mine drainage, transport and sequestration of contaminants, sequestration of carbon dioxide at depth in the earth, ore deposition, and metamorphism. On a global basis, mineral weathering kinetics are also involved in the long-term sink for CO2 in the atmosphere:CaSiO3+CO2=CaCO3+SiO2(1)MgSiO3+CO2=MgCO3+SiO2(2)These reactions (Urey, 1952) describe the processes that balance the volcanic and metamorphic CO2 production to maintain relatively constant levels of atmospheric CO2 over 105-106 yr timescales. In these equations, Ca- and MgSiO3 represent all calcium- and magnesium-containing silicates. Calcium- and magnesium-silicates at the Earth's surface are predominantly plagioclase feldspars, Ca-Mg-pyroxenes, amphiboles, and phyllosilicates, Ca-Mg orthosilicates. Although dissolution of the other main rock-forming mineral class, carbonate minerals, does not draw down CO2 from the atmosphere over geologic timescales, carbonate dissolution is globally important in controlling river and ground water chemistry.Despite the importance of mineral dissolution, field weathering rates are generally observed to be up to five orders of magnitude slower than laboratory dissolution rates (White, 1995), and the reason for this discrepancy remains a puzzle. For example, mean lifetimes of 1 mm spheres of rock-forming minerals calculated from measured rate data following Lasaga (1984) are much smaller than the mean half-life of sedimentary rocks (600 My, Garrels and Mackenzie, 1971). As pointed out by others ( Velbel, 1993a), the order of stability of minerals calculated from measured dissolution kinetics ( Table 1) generally follow weathering trends observed in the field (e.g., Goldich, 1938) with some exceptions. Some have suggested that quantitative prediction of field rates will be near-impossible, although such rate trends may be predictable ( Casey et al., 1993a). For studies with mineral substrates identical between laboratory and field, however, the discrepancy between field and laboratory rate estimate is generally on the order of one to two orders of magnitude (e.g., Schnoor, 1990; Swoboda-Colberg and Drever, 1993; White and Brantley, in press). Some of the discrepancy may be related to factors in the field that have not been well mimicked in laboratory systems ( White (in press) see Chapter 5.05). For example, to extrapolate mineral reaction rates from one system to another, the following variables must be understood: (i) mechanism of dissolution, (ii) reactive surface area, (iii) mineral composition, (iv) temperature of dissolution, (v) chemistry of dissolving solutions, (vi) chemical affinity of dissolving solutions, (vii) duration of dissolution, (viii) hydrologic parameters, and (ix) biological factors. In this chapter, general techniques of measurement of dissolution and precipitation rates of rock-forming silicates and carbonates are discussed, and then, seven of these nine factors are discussed sequentially. A full discussion of the biological effects (discussed by Berner et al. in Chapter 5.06) and hydrological parameters are outside the scope of this chapter.