Reaction rate-surface area relationships during the early stages of weathering. II. Data on eight additional feldspars

Dissolution rate data for eight specimens of feldspars have been obtained as functions of mass and exposed surface area in pH 2 and 3 solutions. These measurements suggest that reaction rates are not related to specific surface areas (m2/g) in any simple fashion, and that the relation expresses itself differently for the alkali feldspars and the plagioclases. The alkali feldspars display rate minima at intermediate size fractions, suggesting that 1) defects associated with exsolution lamella are the primary loci for reaction, and 2) these minerals cleave preferentially along these boundaries, yielding a net reduction in the number of available reaction sites in the finer grain sized fractions. Plagioclase feldspars, as a group, do not display the pronounced rate minima at intermediate size fractions. However, the finer grain-sized fractions do exhibit reduced specific reaction rates, suggesting that the primary sites for the dissolution are not uniformly distributed within the host mineral on the scale of several tens of microns. Two independent approaches are employed to obtain estimates of the precision of individual rate determinations. Replicate rate measurements on related samples have average coefficients of variation of about 8%, with a maximum cv of less than 18%. Variance estimates obtained on the specific rate data from the coarsest size fractions of each mineral yield comparable precision estimates. While there is some uncertainty associated with individual rate determinations, the magnitude of this variance is small compared with discrepancies between field and laboratory-based estimates of absolute rates.

[1]  M. Krom,et al.  Dissolution of Pyroxenes and Amphiboles During Weathering , 1980, Science.

[2]  P. Aagaard,et al.  Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions; I, Theoretical considerations , 1982 .

[3]  D. Grandstaff.  Some kinetics of bronzite orthopyroxene dissolution , 1977 .

[4]  R. Berner,et al.  Rate control in dissolution of alkali feldspars—I. Study of residual feldspar grains by X-ray photoelectron spectroscopy , 1976 .

[5]  M. J. Wilson,et al.  Experimental etching of a microcline perthite and implications regarding natural weathering , 1980 .

[6]  P. Buseck,et al.  High Resolution Electron Microscopy of Feldspar Weathering , 1980 .

[7]  M. Velbel.  Natural weathering mechanisms of almandine garnet , 1984 .

[8]  G. R. Holdren,et al.  Mechanism of feldspar weathering: Some observational evidence , 1977 .

[9]  R. Berner,et al.  X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering , 1983 .

[10]  R. Stallard,et al.  Dissolution at dislocation etch pits in quartz , 1986 .

[11]  Lei Chou,et al.  Steady-state kinetics and dissolution mechanisms of albite , 1985 .

[12]  K. Knauss,et al.  Dependence of albite dissolution kinetics on ph and time at 25°c and 70°c , 1986 .

[13]  J. Strickland.  A practical hand-book of seawater analysis , 1972 .

[14]  P. Fung,et al.  Surface studies of feldspar dissolution using surface replication combined with electron microscopic and spectroscopic techniques , 1982 .

[15]  R. Berner,et al.  Mechanism of pyroxene and amphibole weathering-I. Experimental studies of iron-free minerals , 1981 .

[16]  P. Aagaard,et al.  Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants, effective surface area, and the hydrolysis of feldspar , 1984 .

[17]  G. R. Holdren,et al.  Mechanism of feldspar weathering—I. Experimental studies , 1979 .

[18]  G. R. Holdren,et al.  Mechanism of feldspar weathering—II. Observations of feldspars from soils , 1979 .

[19]  M. Velbel,et al.  Geochemical mass balances and weathering rates in forested watersheds of the Southern Blue Ridge: Solving more equations in more unknowns through incorporation of rare earth elements , 1985 .

[20]  A. Lasaga,et al.  Surface chemistry, etch pits and mineral-water reactions , 1986 .

[21]  G. Westbrook,et al.  Cross section of an accretionary wedge: Barbados Ridge complex , 1988 .

[22]  A. Lasaga.  Chemical kinetics of water‐rock interactions , 1984 .

[23]  G. R. Holdren,et al.  Reaction rate-surface area relationships during the early stages of weathering—I. Initial observations , 1985 .

[24]  G. R. Holdren,et al.  pH dependent changes in the rates and stoichiometry of dissolution of an alkali feldspar at room temperature , 1985 .

[25]  R. Wollast.  Kinetics of the alteration of K-feldspar in buffered solutions at low temperature , 1967 .

[26]  R. Berner,et al.  Mechanism of pyroxene and amphibole weathering; II, Observations of soil grains , 1982 .

[27]  D. Grandstaff.  Changes in surface area and morphology and the mechanism of forsterite dissolution , 1978 .

[28]  M. Wilson.  CHEMICAL WEATHERING OF SOME PRIMARY ROCK‐FORMING MINERALS , 1975 .

[29]  T. Pačes.  Rate constants of dissolution derived from the measurements of mass balance in hydrological catchments , 1983 .

[30]  S. S. Goldich.  A Study in Rock-Weathering , 1938, The Journal of Geology.

[31]  A. Lasaga.  Rate laws of chemical reactions , 1981 .

[32]  T. Parsons,et al.  A practical handbook of seawater analysis , 1968 .

[33]  Lei Chou,et al.  Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor , 1984 .