Geology Reference
In-Depth Information
After Na
+
, Mg
2+
is the most abundant cation in seawater
(Table 4.3) and is recovered from it industrially. Although
Ca
2+
is less abundant in seawater, Ca salts (carbonate
and sulfate) are among the first to precipitate from sea-
water during evaporation; few evaporite sequences
include magnesium minerals, which appear only at an
advanced stage of evaporation. Most of the calcium in
sediments, however, occurs in biogenic limestone.
Box 9.2 Lithium, beryllium and boron
Lithium, beryllium and boron are chemically some-
what different from the other members of their
respective groups. For example, except for the differ-
ence in valency, beryllium has more in common
chemically with aluminium than with magnesium or
calcium. the reason for this
diagonal relationship
is
that beryllium's ionic potential is much closer to
that of aluminium than to the other alkaline earth
metals (see Figure 9.2.1), leading to similar, rel-
atively covalent bonding behaviour and similar crys-
tal chemistry. Both Be and al form unreactive, very
hard oxides that are chemically amphoteric (Box 8.1).
Both metals resist acid attack and oxidation - unlike
Mg, for example - owing to the formation of a sur-
face film of oxide.
For the same reason boron has much in common
with silicon. Both are semiconducting metalloids
rather than true metals. B
2
O
3
is an acidic oxide like
SiO
2
. the diagonal relationship between lithium
and magnesium, although less pronounced, means
that Li is found mainly in the Mg sites in silicate
minerals.
Li, Be and B also differ from other elements in their
groups in having anomalously low abundance, in the
earth and in the solar system (as explained in
Chapter 11). For instance, Li is a trace element in most
rocks, whereas Na and K are major elements.
Aluminium
Aluminium
is the commonest metallic element in the
crust (Figure 11.7). Its malleability, ductility, electrical
conductivity, chemical stability and low density make
it an ideal metal for many industrial and domestic
uses. The oxide, alumina (Al
2
O
3
), is
amphoteric
. It forms the extremely hard,
refractory mineral
corundum
(hard-
ness = 9), which is widely used as an abra-
sive (emery) and in ceramics. The
gemstones ruby and sapphire are coloured
varieties of corundum.
The mobility of aluminium in weathering depends
upon the pH of the solution. Al
2
O
3
is extremely insoluble
in the majority of groundwaters, whose pHs tend to lie
in the range 5-6 (Figure 4.2). During the weathering of a
granite under such conditions, Na and Ca can be almost
entirely removed and K, Mg, Fe
3+
and even Si partially
leached, while Al remains immobile. The minerals left
behind are therefore very aluminous, as in the china
clay deposits (consisting largely of kaolinite) associated
with Cornish granites. The weathering of intermediate
volcanic rocks and ash in extreme tropical conditions
can proceed even further and remove most of the silica
too, leaving behind a mixture of aluminium hydroxides
stained by hydrated iron oxides. This material, the chief
industrial source of aluminium, is called
bauxite
.
Contact with rotting vegetation or pollution by acid
rain can, however, make surface waters sufficiently
acid (pH 4 or less) to dissolve alumina. Signs of this can
be seen in soil profiles in temperate forests, where an
upper light-coloured horizon, from which all comp-
onents except silica have been leached by acid solu-
tions, passes down into a lower horizon rich in clay
minerals, in which Al has been re-precipitated through
contact with relatively neutral groundwater at the
water table. Precipitation of Al
3+
(and Fe
3+
) in this way
is an example of
hydrolysis
(Box 9.3).
B
C
Al
Si
Ga
In Sn
Ge
Symbol
lonic
potential
nm
-1
Li
1. 2
Be
5.7
B
15.0
Na
0.8
Mg
2.5
Al
4.9
Si
11.8
K
0.6
Ca
1. 7
Ga
4.3
Ge
8.3
Figure 9.2.1
a condensed version of the periodic
table, illustrating the diagonal similarity in ionic
potential values for first two rows.
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