%
200
180
160
140
120
100
90
80
70
60
50
40
30
20
10
Sample numbers
J1
J2
J3
J4
J5
J6
J7
CaO
MgO
Na 2 O
K 2 O
SiO 2
Al 2 O 3
Fe 2 O 3
TiO 2
Fig. 3.13 Chemical analyses
of changes in the chemical
composition of samples
representing progressive
weathering of an amphibolite
compared to an unweathered
sample. The stable (less
soluble) elements (Ti, Fe and
Al) are enriched while there is
a strong depletion of Na, Mg
and Ca. Potassium is depleted
but is adsorbed on clay
minerals in the soil
8.0
6.0
0.0
4.0
2.0
–4.0
–3.0
–2.0
–1.0
0.0
Quartz
saturation
Amorphous
silica saturation
Log aSiO 2 (aq)
Log
aK + /aH
+
Illite
Gibbsite Kaolinite
Smectite
Microcline
Mixed
layer
clay
Fig. 3.14 Activity diagram
showing the stability of some
minerals as a function of silica
and the activity ratio (after
Aagaard and Helgeson 1982).
Weathering reactions are
characterised by reduction in
both these two parameters
towards the lower left of the
diagram
3 Sedimentary Geochemistry
109