Weathering rates 261 silicate minerals

Lasaga A. C. (1995) Fundamental approaches in describing mineral dissolution and precipitation rates. In Chemical Weathering Rates ( Silicate Minerals (eds. A. F. White and... 

White A. F. and Brantley S. L. (2003) The effect of time on the weathering of silicate minerals why to weathering rates differ in the laboratory and field Chem. Geol. (in press). 

Silicon is mobilized by the weathering of silicate minerals, mainly feldspars (see eqn. 4.14), and is transported in natural waters at near-neutral pH as undissociated silicic acid (H4Si04), an oxyanion (Fig. 5.2). Silicate minerals weather slowly, such that rates of input—and concentrations—of silicon in most freshwaters are quite low. Despite this, where silicates are the main component of bedrock or soil, H4Si04 can be a significant dissolved component of freshwater. 

Wollast, R., and L. Chou (1988), Rate Control of Weathering of Silicate Minerals at Room Temperature and Pressure, in A. Lerman and M. Meybeck, Eds., Physical and Chemical Weathering in Geochemical Cycles, Kluwer Academic Publishers, Dordrecht, Netherlands, pp. 11-32. 

Johnson, N. M., G. E. Likens, F. H. Borman, and R. S. Pierce (1968). Rate of chemical weathering of silicate minerals in New Hampshire. Geochim. Cosmochim. Acta 32, 531-545. 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

White, A. F., 1995, Chemical weathering rates of silicate minerals in soils. Reviews in Mineralogy 31,407 161. 

Blum, A.E. Stillings, L.L. 1995. Felsdpar dissolution kinetics. In White, A.F. Brantley, S.L. (ed.), Reviews in mineralogy, 31 Chemical weathering rates of silicate minerals, Mineralogical Society of America, USA, 291-352. 

These reactions, however, are complex and generally proceed through a series of reaction steps. The rate of weathering of silicates may vary considerably depending on the arrangement of the silicon tetrahedra in the mineral and on the nature of the cations. 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Estimate on the basis of results on experimental weathering rates and the number of functional groups (or central ions) per unit area, how long it takes to dissolve one "monomolecuar" layer of a representative silicate mineral. 

The importance of "parabolic kinetics" in laboratory studies of mineral dissolution has varied as interpretations of the underlying rate-controlling mechanism have changed. Much of the research on silicate mineral weathering undertaken in the past decade or so served to test various hypotheses for the origin of parabolic kinetics. 

Bums, R.G. (1993) Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars. Geochim. Cosmochim. Acta 57 4555-4574... 

Hochella, M. F. Banfield, J. F. 1985. Chemical weathering of silicates in nature A microscopic perspective with theoretical considerations. In White, A. F. Brantely, S. L. (eds) Chemical Weathering Rates of Silicate Minerals. 31, Mineralogical Society of America, Washington, DC, 353-406. 

Velbel, M. A. 1993. Constancy of silicate-mineral weathering-rate ratios between natural and experimental weathering Implications for hydrologic control of differences in absolute rates. Chemical Geology, 105, 89-99. 

Lake level could also influence silica levels. For example, the decline in lake level resulted in a large loss of surface area and decreased the amount of contact between lake water and sandy littoral areas that contain weath-erable silicate minerals. Rates of weathering are usually enhanced by acidification and would help to explain the interbasin differences observed at pH 4.7. However, weathering rates of LRL sediment are unknown (see Sediment Processes section). In addition, although the differences between the basins were significant, they were small (0.01-0.03 mg of Si02 per liter) and may be accounted for by small differences in hydrological factors. 

Casey, W. H. Westrich, H. R. (1991) General relations among weathering rates of some silicate minerals. Geol. Soc. Amer., Ann. Meet., Abstr., 23, A258. 

Berner, R. A. (1995). Chemical weathering and its effect on atmospheric CO2 and climate. In Chemical Weathering Rates in Silicate Minerals, ed. A. F. White S. L. Brantley. Washington DC Mineralogical Society of America, pp. 565-83. 

---