Geoscience Reference
In-Depth Information
KAl
2
AlSi
3
O
10
(OH)
2
muscovite
K(Fe,Mg)
3
AlSi
3
O
10
(OH)
2
biotite
+(Al
3+
, Mg
2+
)
-K
+
-K
+
K
x
Al
2
Al
1-x
Si
3
O
10
(OH)
2
illite
(Mg,Al)
3
(OH)
6
(Fe,Mg)
3
(AlSi)
4
O
10
(OH)
2
chlorite
-K
+
-K
+
+H
+
-H
+
-K
+
+Na
+
Mg
x
A
2
(AlSi)
4
O
10
(OH)
2
Dioctahedral vermiculite
,
K
1-x
Mg
x
(Fe,Mg)
3
AlSi
3
O
10
(OH)
2
Trioctahedral vermiculite
-Mg
2+
+Mg
2+
+Na
+
+(Na
+
,Ca
2+
)
FeO(OH)
goethite
-Fe
3+
(Na,Ca)
x+y
(Al
2-x
Mg
x
)(Si
4-y
Al
y
)O
10
(OH)
2
smectite
+H
+
-Si(OH)
4
-(Na
+
,Ca
2+
,Mg
2+
)
Al
2
Si
3
O
5
(OH)
4
Kaolinite
+H
+
-Si(OH)
4
Al(OH)
3
gibbsite
Fig. 2.1
Possible chemical weathering pathways of muscovite and biotite
2.2 Weathering
Weathering of subsurface solid phases occurs as a result of their direct interaction
with liquid phases, which may also in turn be affected by the gaseous environment.
Examples of weathering processes include reactions that convert primary minerals
such as quartz and clays into metal oxides and metal hydroxides.
The major chemical weathering agent in the subsurface is water, which can act
as either a weak acid or a base. Oxygen can oxidize organic hydrocarbons and a
variety of metals that include Fe
2+
and S
2-
. Carbon dioxide can be transformed to
function as an inorganic acid (e.g., HCO
3
-
) or as an organic acid (e.g., HCOOH),
and the conjugate bases are often strong ligands that complex metals.
A possible chemical weathering process of two primary minerals, muscovite
and biotite, and their various mineral products is presented in Fig.
2.1
.
2.2.1 Dissolution and Precipitation
Dissolution and precipitation in the subsurface are controlled by the properties of
the solid phases, by the chemistry of infiltrating water, by the presence of a gas
