Geology Reference
In-Depth Information
9
SOME GEOLOGICALLY IMPORTANT
ELEMENTS
Major and trace elements
Chemical elements are of interest to geoscientists for
various reasons. Some, such as silicon (Si) and iron (Fe),
are so abundant that their chemical properties govern
the behaviour of geological materials. Less abundant
elements like rubidium (Rb) and strontium (Sr), though
they participate more passively in geological processes,
may nonetheless help us to understand how such pro-
cesses work. Other elements (such as chromium, Cr, neo-
dymium, Nd, or uranium, U) have important commercial
uses or environmental impacts (e.g. radon in Box 9.9).
The purpose of this chapter is to introduce the reader
to the geochemistry of the more important chemical
elements, to show how the Periodic Table can be used to
predict element behaviour in the Earth and on its surface
as well as in the laboratory, and to illustrate what these
elements can tell us about Earth processes. Chapter 10
outlines the additional information that be learned from
the study of the
isotopes
of naturally occurring elements.
In terms of their abundance in geological materials,
elements can be divided into two classes:
major elements
and
trace elements
.
Major elements
These elements, which include Si, Al, Mg and Na, have
concentrations in most geological materials in excess of
0.1%. They are essential constituents of rock-forming
minerals. Major element concentrations in silicate min-
erals are usually expressed in terms of
oxide percentages
(Box 8.3; Figure 9.1). The term
minor element
is some-
times applied to the less abundant major elements, such
as manganese (Mn) and phosphorus (P), with oxide
concentrations below 1%.
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