41 by Shotyk and Nesbitt 1992 and Welch and Ullman 1996 that the dissolved
polyvalent cations mainly Al form metal-organic complexes thereby increase the concentration gradients of these cations within solution – rock particle interfaces,
enhancing diffusion rate of these cations from the rock particles to bulk solution. In this present experiment, this dissolution mechanism occurred more intensively for
dry- than wet-milled rocks.
3.3.2. Effects of Milling on Dissolution Kinetics
The effects of milling time and milling condition on the dissolution kinetics of silicate rocks may be summarized as follows, and is based on the results
presented in Table 3.1, Figures 3.1 and 3.2:
1. Milling increased the proportion of rocks dissolved E
t
with the increase produced by dry milling being greater than for wet milling.
2. Milling increased the quantity of rapidly soluble rock Eo and the amount of reactive sites on the surfaces of rock particles as indicated by coefficient a,
with the increases due to dry milling being higher than for wet milling Table 3.1.
3. Milling increased the dissolution rate R
t
Figure 3.2. 4. There was no systematic trend of reaction order
n
with increasing milling time, apart from values for original rocks T0 mostly being larger than for milled
rocks T10 – T120. The values for mafic rocks were mostly lower 0.30 – 0.50 than those for felsic rocks 0.41 – 0.71.
As shown in Figure 3.1, the percentage dissolution of ground basalt and dolerite mafic rocks was greater than for ground gneiss and K-feldspar felsic
rocks. For example the E
t
values at t = 56 days for initially milled-basalt and dolerite were about 10 and were increased to about 35 by dry milling and 25
by wet milling for 120 min. The E
t
values at t = 56 days for initially milled-gneiss and K-feldspar were about 3 , and were increased to about 22 by dry milling
and 12 by wet milling for 120 min. For all rocks most of the increased dissolution produced by milling was due to the large increase in rapidly dissolved rock Eo.
In addition to the increase in initial dissolution Eo milling greatly increased dissolution rate R
t
Figure 3.2 during the early stages of dissolution within 1 h of dissolution up to values of about 15 d for milled mafic rocks and 6 – 10 d for
42 milled felsic rocks. The dissolution rates decreased greatly with the increase of
dissolution time, trending towards a constant dissolution rate. This result may be interpreted as indicating that by 56 days of dissolution of only crystalline minerals
was occurring for both dry- and wet-milled samples. It is therefore concluded that the reduced particle size, the increased effective
surface area, and the increased abundance of amorphous and disordered constituents due to milling resulted in an increase in exchangeable alkali and other cations. These
changes also resulted in an increase in the dissolution rate R
t
, the proportion of rapidly soluble rock Eo, and the amount of reactive sites on the surface of particles
a. The changes in dissolution due to milling were greater for dry than for wet milling.
3.3.3. Elemental Dissolution of Silicate Rocks
