p18ctqouf5gk614re1k6k1lc31s0l4.pdf

154 Rates of Chemical Weathering
36
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28 UNTREATED
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bI 16
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00 20 40 60 80
Time (days)
Figure 7.6. Silica release during the dissolution of untreated, sonified, and etched enstatite
in buffer solution at pH = 6, T = 293 K. [From Schott et al. (1981), with permission.
However, Berner et al. (1985) suggest that the contradictions "may be
illusory." They point out that limited incongruency could be accomodated
with the XPS data sets Chou and Wollast (1984) dispute. Berner et al.
(1985) suggest that spatially inhomogeneous incongruent dissolution along
tubes (e.g., etch pits that penetrate deep into the interiors of mineral
grains) could reconcile XPS measurements (to which the interior surfaces
of tubes would not be accessible) with the incongruency and material
balance requirements of laboratory studies, while still allowing surfacereaction
control. Chou and Wollast (1985) tend to agree with Berner et al.
(1985) concerning the role of dissolution-void interiors in resolving the
above differences.
Structural Mineral Distortions and Hyperfine Particles. The early.
high-rate stage observed for dissolution of feldspars and ferromagnesian
minerals has also been ascribed to a structurally distorted or strained outer
layer of the mineral, or to large quantities of hyperfine particles adhering
electrostatically to the outer surface of the mineral (Velbel, 1986). An
example of how pretreatment of minerals affects silica release from
enstatite is shown in Fig. 7.6. The rate of silica dissolution of untreated and
sonified samples was fast initially and then decreased with time, resulting in
parabolic kinetics. This behavior was ascribed to initial rapid dissolution of
uitrafine, supersoluble particles adhering to large grains that were produced
during grinding (Holdren and Berner, 1979; Schott et al., 1981).