SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 154

Alkali metals

100.0

Si
Al
Mg
K

10.0

The alkali metals constitute the first column (Group la)
on the left-hand side of the Periodic Table. Their key
chemical features are as follows:

1. 0

Mn

0.1 1000

(a) They are strongly electropositive ele-
ments (Figure 6.3); their compounds are
characteristically ionic and they form
basic oxides.
(b) The M + cations are large (Figure 9.1.1) and
can be accommodated only in relatively
large cation sites, such as the 'A-site' in
amphiboles and micas (Chapter 8) and in
framework silicate minerals. Feldspar is
the main host of these elements in most rocks.
(c) They are very soluble in aqueous fluids, and are
among the first elements to be dissolved during
weathering. Na and K are important constituents
of seawater, and evaporite deposits provide the
main industrial source.

H

Ba
Ni
Rb
La
Pb
Sn
U

Li Be

0.01 100

Na
K Ca
Rb Sr
Cs
Fr

Mg

10

1000 1

Ba

100 Ag

Bi

0.1

10

Au
Pd

1

0.1

lr

Major elements Tr ace elements

Sodium (Na) and potassium (K) are important con-
stituents of feldspar, amphibole and mica and there-
fore are found as major elements in most rocks of the
continental crust. Potassium is an important plant
nutrient, hence its widespread use in fertilizers.
Rubidium (Rb) and caesium (Cs, spelt cesium in North
America), on the other hand, are too scarce to form
their own minerals and occur as trace elements, enter-
ing rock-forming silicates only where they can substi-
tute for K + ions (e.g. in alkali feldspar).
The large ionic radii of K + , Rb + , Cs + and to a lesser
extent Na + (Figure 9.1.1) lead to their exclusion from
dense ferromagnesian minerals like olivine and pyrox-
ene. Such minerals, with calcic plagioclase, are the first to
crystallize from basic magmas, and because the alkali
metals are excluded they remain in the melt. As the vol-
ume of melt decreases with advancing crystallization, the
concentration of dissolved K, Rb and Cs in the remaining
melt increases. A series of lava flows tapping a magma
chamber at successive stages in its crystallization would
therefore exhibit increasing concentrations of these ele-
ments in later flows. In plutonic rocks they are enriched
in 'late-stage' granites and pegmatites. Elements like
these, whose exclusion from the main igneous minerals
leads to their concentration in late-stage residual mag-
mas, are called incompatible elements (Box 9.1).

* Major elements are usually expressed as mass
percent oxide (Box 8.3)

Figure 9.1 How units of element concentration compare.
The positions of element symbols illustrate the average
concentration of the selected element in the continental crust.

Trace elements

Trace elements, such as rubidium (Rb) and zinc (Zn),
have concentrations in most geological materials too
low - usually less than 0.1% - for them to influence
which minerals crystallize. They mostly occur as dis-
solved 'impurities' in major rock-forming minerals,
although some - such as zirconium (Zr) - may form
separate accessory minerals (e.g. zircon, ZrSiO 4 ). Trace
element concentrations in rocks are usually expressed
in parts per million ( ppm = 1 μg g −1 ) of the element (not
the oxide) or in parts per billion (ppb = 1 ng g −1 ) - see
Figure 9.1.
The distinction between major and trace elements
has to be applied in a flexible way, because the same
element can be a major element in one rock type (potas-
sium in granite, for example) and a trace element in
another (potassium in peridotite).