Geoscience Reference
In-Depth Information
O
Si
O
O
O
O
O
Si
O
Si
O
O
O
O
Figure 1.10
SiO
4
tetrahedra, which are the building blocks of silicate structures and their polymerization.
Aluminum similarly forms AlO
4
tetrahedra.
2. Single-chain silicates: these are the pyroxenes, which fall into two groups with two
different crystallographic systems; orthopyroxenes, such as enstatite Mg
2
Si
2
O
6
, and
clinopyroxenes, such as diopside CaMgSi
2
O
6
.
3. Double-chain silicates: amphiboles, such as tremolite Ca
2
Mg
5
Si
8
O
22
(OH)
2
or horn-
blende Ca
2
Mg
4
Al
2
Si
7
O
22
(OH)
2
. The formation of
these hydroxylated minerals
requires some degree of water pressure.
4. Sheet silicates: micas and clay minerals usually containing aluminum, potassium, and
smaller ions such as Fe
2
+
and Mg
2
+
. A distinction is drawn between di-octahedral micas
like muscovite (common white mica) K
2
Al
6
Si
6
O
20
(OH)
4
and tri-octahedral micas like
biotite (ordinary black mica) K
2
Mg
6
Al
2
Si
6
O
20
(OH)
4
, the difference being the propor-
tions of 2
+
and 3
+
cations and therefore site occupancy. This family is extremely
diverse.
5. Framework silicates: these silicates are interconnected at each of their apexes. This
family includes quartz SiO
2
and the feldspars, the most important of which are albite
NaAlSi
3
O
8
, anorthite CaAl
2
Si
2
O
8
, and the various potassium feldspars whose formula
is KAlSi
3
O
8
.
Other significant minerals include iron oxides and titanium oxides, which are commonly
cubic ionic solids such as magnetite Fe
3
O
4
and ilmenite FeTiO
3
. Corundum is the oxide of
aluminum Al
2
O
3
. Calcium carbonate (calcite, aragonite) and magnesium carbonate (mag-
nesite), formed by stacking of Ca
2
+
or Mg
2
+
ions, contain small carbonate groupings
CO
2
3
able to rotate around the axis of symmetry.
The minerals cited above are only examples, albeit important ones, but alone they fail to
provide a sufficiently precise representation of the chemical diversity of rocks. Elements
are located within minerals at sites characterized by the number of oxygen atoms that they
have as their immediate neighbors. We have already come across the tetravalent silicon
ion Si
4
+
(i.e. carrying four positive charges) at a tetrahedral site and said that it could be
replaced by the trivalent aluminum ion Al
3
+
. Electrical neutrality is maintained through
paired substitutions such as Al
3
+
Al
3
+
substituting for Si
4
+
Mg
2
+
. Such substitution is pos-
sible and even commonplace in pyroxenes, amphiboles, micas, and feldspars as the two
ions have similar ionic radii (0.39 and 0.26 Å, respectively) and similar electrical charges.
