Geology Reference
In-Depth Information
Box 8.1 Acidic and basic oxides
the composition of any silicate material can be repre-
sented as a combination of various metallic oxides such as
MgO and K
2
O combined with silicon dioxide (silica).
Forsterite, for example, can be regarded as the result of
reacting two molecules of MgO with one of SiO
2
:
(
)
CO +OH+H CO +H
2
−
+
→
−
+
3
(cf. equations 4.19 and 4.21) resulting in an acidic
solution. Similar reactions involving the oxides of sulfur
produced by coal-burning power stations are environ-
mentally important in leading to acid rain (see O'Neill,
1998). In the context of silicates, this category of
acidic
oxides
includes the oxides of silicon and phosphorus
(Figure 9.4).
2MgO+SiO
→
Mg SiO
(8.1.1)
2
2
4
the character of a silicate depends upon the proportions
in which different oxides combine. Oxides can be sub-
divided into three types:
Amphoteric oxides
Amphoteric oxides and hydroxides share the property of
being able to behave as acids (if reacting with a strong
base)
and
bases (with a strong acid):
Basic oxides
When an
ionic oxide
dissolves in water, the O
2−
ion released
reacts with a water molecule to form two hydroxyl ions:
(
)
+
+→ →+
+
2
−
−
Na OHO2Na OHO Na
+
+
2OH
2
2
2
(
)
Al OH +3HCl lCl+3H O
3
→
3
2
the production of Oh
−
ions removes free h
+
ions from the
solution, increasing its ph and making it
basic
(appendix
B). For this reason the oxides of the electropositive metals
are described as
basic oxides
(Figure 9.4).
base
acid
salt
water
(
)
Al OH +NaOH aAlO +2HO
3
→
2
2
acid
base
salt
(sodium
aluminate)
water
Acidic oxides
a
covalent oxide
dissolved in water, instead of producing
Oh
−
ions, will remove them from solution:
Other elements forming amphoteric oxides are shown in
Figure 9.4.
Table 8.1
Silicate polymers
Structural type
p
Z:O
Example mineral
Formula
Orthosilicate (monomer)
4
1 : 4
Forsterite (olivine)
Mg
2
[SiO
4
]
Dimer
3
1 : 3.5
Melilite
Ca
2
Mg[Si
2
O
7
]
ring silicate
2
1 : 3
Beryl
Be
3
al
2
[Si
6
O
18
]
Chain silicates
pyroxene
2
1 : 3
Diopside (pyroxene)
CaMg[Si
2
O
6
]
amphibole
1.5
1 : 2.75
tremolite (amphibole)
Ca
2
Mg
5
[Si
8
O
22
](Oh)
2
Sheet silicate
1
1 : 2.5
Muscovite (mica)
Kal
2
[alSi
3
O
10
](Oh)
2
Framework silicate
0
1 : 2
Orthoclase (feldspar)
K[alSi
3
O
8
]
Dimer silicates
Silicates built of isolated SiO
4
tetrahedra are commonly
known as
orthosilicates
. They include other minerals
like garnet (e.g. grossular, Ca
3
Al
2
Si
3
O
12
), zircon (ZrSiO
4
)
and topaz (Al
2
SiO
4
F
2
). Notice that in each of these for-
mulae the ratio of Si to O is 1:4, a universal characteris-
tic of orthosilicates. All of the oxygen atoms in these
structures are non-bridging, so they share the value
p
= 4 (Table 8.1).
The structures of a few minerals involve pairs of SiO
4
tetra-
hedra linked through a single bridging oxygen (
p
= 3). The
formula of this dimeric group, consisting of two SiO
4
groups
less one oxygen (=Si
2
O
7
), can be seen in melilite (Ca
2
MgSi
2
O
7
,
Figure 8.1b). The commoner mineral epidote contains both
single (SiO
4
) and double (Si
2
O
7
) tetrahedral groups.
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