THE ELEMENTS IN THE UNIVERSE - Chemical Fundamentals of Geology and Environmental Geoscience - page 215

Atmophile

N

Atmosphere and
hydrosphere

He Ne Ar Kr Xe

Lithophile

O

Li Be B F

Si

Na Mg Al

HClBrI

K
Sc
Rb

Ca

Crust, mantle
and stony
meteorites

Ti
Sr Y rNb
Hf

Cs Ba REE U Th

Core and
iron meteorites

C

P W Ta

Fe Ni

Ga

VCrMnZn

Co Sn

Ru
Os

Rh Pd Ge As

Sb
Re

Se

Sulfide minerals

S

Ir Pt Au Mo Te

Cu Ag Cd In
Hg Tl Pb Bi

Chalcophile

Figure 11.4 Element affinities in the Earth and in meteorites. Areas of overlap show elements common to two or more
phases. Larger lettering indicates a major element. Elements found principally in the metal phase are shown italicized.
( REE - Figure 11.2.)

O 2 and H 2 O gases in the Earth's atmosphere). Another
prominent example is iron, which exhibits lithophile,
siderophile and chalcophile tendencies and therefore
lies in the area of overlap between all three of these
fields in Figure 11.4.

Magnesium and silicon, the elements that make up
the bulk of the silicate minerals in meteorites and plan-
ets, form a 'moderately refractory' category between
these two extremes. The major siderophile elements
fall within the same range of volatility (Figure 11.5).
The remaining lithophile and chalcophile elements
are volatile in varying degrees. We can divide them
into moderately volatile (e.g. Na, Mn, Cu, F, S) and
very volatile (C, Cl, Pb, Cd, Hg) categories, as shown in
Figure 11.5. The atmophile elements can be considered
as a third, 'most volatile' category.

Volatile versus refractory

In considering the development of the Solar System, it
is also useful to subdivide chemical elements accord-
ing to their volatility. Volatile elements are those that
become gaseous at relatively low temperatures. In cos-
mochemical terms they include not only the atmophile
elements hydrogen, helium (and other inert gases) and
nitrogen that are gases at room temperature, but also
such elements as cadmium (Cd), lead (Pb), sulfur (S)
and most of the alkali metals. Refractory elements, on
the other hand, are those that remain solid up to very
high temperatures. The most refractory elements are
the platinum metals (like iridium), and we also include
in this category elements like calcium, aluminium and
titanium that form highly refractory oxide or silicate
compounds (such as the minerals perovskite, CaTiO 3
and anorthite, CaAl 2 Si 2 O 8 ).

Element fractionation in the Solar System

It has long been known that the planets orbiting our
Sun vary considerably in composition, evidence that,
during the development of the Solar System, the ele-
ments have been chemically sorted or fractionated .
Because metal, silicate and gas phases themselves
differ in density, planetary scientists are able to esti-
mate the proportions of these materials present in
the planets (whose mean densities can be determined
from astronomical measurements). As one can see in
Figure 11.6, the planets differ considerably in their