Geology Reference
In-Depth Information
geochemist V.M. Goldschmidt devised the following
Big bang
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(a) lithophile elements: those concentrated into the
silicate phase (from the Greek lithos , meaning
(b) siderophile elements: those preferring the metal
phase (from the Greek sideros , meaning 'iron');
(c) chalcophile elements: those like copper which
concentrate in the sulfide phase (from the Greek
chalcos , meaning 'copper');
(d) atmophile elements: gaseous elements (from the
Greek atmos , meaning 'steam' or 'vapour').
Stellar fusion
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Neutron capture
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10 2
10 0
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How the elements are divided between these cat-
egories is illustrated in Figure 11.4 and Plate 7. Such a
compilation involves compromise, and one author's
version may differ slightly from another's. Among the
metallic elements there is a significant correlation with
electronegativity (cf. Box 9.9): the metals that are excl-
usively lithophile (excluding B and Si) have electron-
egativities below 1.7, most chalcophile metals have
electronegativities between 1.8 and 2.3, and the most
siderophile metals are those with electronegativities of
2.2 and above. Goldschmidt's concept is very useful in
understanding in what form elements occur in Solar-
System matter, in ore deposits, or for that matter in a
smelter. For example, the siderophile character of irid-
ium (Ir) means that nearly all of the Earth's Ir inven-
tory is locked away in the metallic core (the same
incidentally being true of gold) and its concentration
in crustal rocks is extremely low (see Figure  11.2:
Z Ir = 77). Consequently most of the iridium detected on
the Earth's surface, in deep-sea sediments for instance,
has been introduced there as a constituent of incoming
meteoritic dust; some iron meteorites contain as much
as 20 ppm Ir, 20,000 times higher than average levels in
crustal rocks. This provides a means of estimating the
annual influx of iron meteorites to the Earth's surface.
Positive Ir anomalies are also characteristic of clays
associated in time with the Cretaceous-Tertiary extinc-
tion, one of the factors suggesting that a major impact
event occurred around that time.
Some elements exhibit more than one affinity, neces-
sitating areas of overlap in Figure  11.4 and multiple
colours in Plate 7. For example, oxygen is considered
to be both lithophile - being a major constituent of all
silicates, as explained in Chapter 8 - and atmophile (as
Atomic number Z
Figure 11.3 The Solar-System abundance curve (Figure 11.2)
showing the domains of various nucleosynthetic processes.
out into the interstellar medium, to be eventually
incorporated into new generations of stars. Present
element abundances (Figure 11.2) reflect recycling
of matter through successive generations of stars,
each one adding its own contribution to the overall
accumulation of heavy elements in the universe.
Figure 11.3 summarizes the contribution of these dif-
ferent processes to the current inventory of chemical
elements in the universe. As Hutchison (1983) pointed
out, 'we, each one of us, have part of a star inside us'.
More detailed accounts of stellar nucleosynthesis can
be found in the topics by Albarède (2009) and Ferreira
(2006), and in a recent review article by Rauscher and
Patkós (2011).
Elements in the Solar System
Cosmochemical classification
Differentiated meteorites as a group contain three
broad categories of solid material: silicate, metal and
sulfide. Analysis of these phases shows that most
elements have a greater affinity with one of them
than with the others. Magnesium, for example, is over-
whelmingly segregated into silicate phases, whereas
copper is often concentrated in sulfides. The Norwegian
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