The properties of elements - Geochemistry: An Introduction - page 13

Likewise, Mg 2 + ions (0.72 Å) located at octahedral sites of naturally occurring olivine,
i.e. those surrounded by six oxygen atoms, are commonly replaced by Fe 2 + ions (0.61 Å)
or Ni 2 + ions (0.69 Å). Ions of the rare-earth element family, such as the Yb 3 + ytterbium
ion (0.99 Å), may substitute for the Ca 2 + ion (1.00 Å) in clinopyroxenes or amphiboles.
As these continuous substitution phenomena are analogous to those whereby various ions
co-exist in aqueous solutions, the term solid solution is used.
Not all solid solutions are possible. The larger ions, such as the alkali metals (K + ,Rb + )
or the alkaline-earth metals (Sr 2 + ,Ba 2 + ) of the higher periods, have ionic radii that are too
big for them to fit readily into the common silicate minerals. Ions with different charges
manage to substitute for major ions if the electrical imbalance can be offset locally: a
(Ca 2 + ) 2 pair may thus be replaced by a Yb 3 + Na + pair in a clinopyroxene, whereas its
replacement by a Th 4 + thorium ion requires the formation of a defect with a high energy
cost. Ions that carry too high a charge or whose ionic radius is too small or too large are
rejected by the lattice of essential minerals and concentrate either in accessory minerals,
such as the phosphates or titanates, or in poorly characterized phases in grain fractures
and interstices. These outcasts are termed incompatible elements. However, this is a rel-
ative concept: potassium and barium are incompatible in the feldspar-free mantle; yet, in
the continental crust, where feldspar is abundant, K and Ba are compatible. The volatile
elements and compounds, such as the rare gases, water, and carbon dioxide, call for some
attention. As long as no gas phase is present, i.e. as long as the concentration of one of the
most abundant volatiles in the solid or liquid in question does not exceed saturation level,
they behave like any other element with varying levels of compatibility. In the absence of
vapor, for example, the inert gases such as helium or argon need not be classified separately
from the other trace elements.
1.4 Geochemical classifications
There are many ways to arrange elements by their geochemical properties. Such practice,
reducing as it does the wide diversity of behavior of elements to limited ranges of behavior,
might be thought pointless, but it does provide an overview of their chemical proper-
ties. The most widespread classification is probably that of Victor Goldschmidt ( Fig. 1.2 ) .
Goldschmidt's scheme rests on the observation by Berzelius, a Swedish eighteenth-century
chemist, that some elements tend to form oxides or carbonates whereas others form
sulfides. The lithophile elements (Na, K, Si, Al, Ti, Mg, Ca) generally concentrate in
the rock-forming minerals of the crust and mantle; the siderophile elements (Fe, Co,
Ni, Pt, Re, Os) have an affinity for iron and therefore concentrate in the Earth's core;
the chalcophile elements (Cu, Ag, Zn, Pb, S) readily form sulfides; the atmophile ele-
ments (O, N, H, and the inert gases) concentrate in the atmosphere. Certain elements
in each group tend to be volatile; K is a more volatile lithophile than either Mg or Ti
(see Chapter 12 ). Refractory elements such as Mg or Cr tend to concentrate in solid
residues.