SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 155

Box 9.1 Incompatible elements

Incompatible elements are those whose cations are not
easily accommodated in the structures of the principal
igneous minerals. Being excluded from these minerals as
magma crystallization progresses, they become progres-
sively more enriched in the diminishing amount of residual
melt, whose disordered, less compact structure accepts
them more readily.
the shaded band in Figure 9.1.1 divides elements
whose ions are readily accommodated in relevant igneous
minerals ( compatible elements ) from elements that are
incompatible. elements whose symbols overlap the band
may fall into either group depending on the minerals that
happen to be crystallizing.
the reason why an incompatible element is excluded
from a mineral's crystal structure depends on its ionic
potential (Figure 7.7):

the behaviour of the rare earth elements (REEs = La to
Lu) and yttrium (Y) is discussed in a later section.

Partition coefficients
a trace element's tendency to behave compatibly or incom-
patibly during magma evolution is expressed numerically by a
number called the partition coefficient , represented by the
symbol K i A , where i identifies a specific element and A speci-
fies the mineral crystallizing. For example, the partition coef-
ficient describing the equilibrium distribution of nickel between
an olivine crystal and a coexisting melt is defined as:

K N olivine

concentration of nickel in olivinecrystal
concentrat

(9.1.1)

=

ii on of nickel in coexisting melt

where the concentrations are given in ppm. Nickel is a
compatible element in olivine and therefore has a part-
ition coefficient greater than 1.00 (typically about 18).
an incompatible element is characterized by partition
coefficient values less than one for all minerals crystalliz-
ing from the melt: rb, for example, has the following part-
ition coefficient values: 0.006 in olivine, 0.04 in
clinopyroxene, 0.25 in amphibole, 0.10 in plagioclase.
an element may behave compatibly in relation to one min-
eral while being incompatible in others. thus Ni is a com-
patible element in olivine but incompatible in plagioclase.
typical partition coefficient values are tabulated in
henderson and henderson (2009).

Large-ion lithophile (LIL) elements
(Ionic potential <40 nm −1 .) these are elements like rb and
Ba, which most crystals exclude because their ions are
simply too large to fit into the cation sites available in the
crystal structure (Box 7.2).

High field-strength (HFS) elements
(Ionic potential ≥40 nm −1 .) an ion like zirconium (Zr 4+ ) has a
radius no bigger than Mg 2+ , yet its high polarizing power and
relatively covalent bonding make it an uncomfortable occu-
pant of a Mg 2+ cation site in a predominantly ionic crystal.

An incompatible element resides
mainly in the melt phase and is
excluded from crystalline minerals.

K j A < 1

(c)

(a)

High field-strength
elements

n+

5

P

NbTa

Incompatible
elements

4

Si Ti

Sn ZrHf U Th

Ga V Sc
Cr Fe 3+

Large-ion
lithophile
elements

Al iv Al vi 3 B Lu Y La

Figure 9.1.1 (a) Compatible and incompat-
ible elements displayed on a graph of cation
charge versus ionic radius. Bold type signifies
major elements. (b) a cartoon illustrating the
distribution of a trace element i that is
compatible in mineral a but not mineral B.
(c) a cartoon illustrating the distribution of a
trace element that is incompatible in
minerals a and B. the density of dots in
(b) and (c) represents the concentration of
element i .

REE

Mn Mg Co Fe 2+
Ni Cu Zn

2

Ca

Be Eu 2+ Sr Pb Ba

Compatible
elements

1

Na K Li Rb Tl C s

0
0.00

0.05 0.10 0.15 0.20

Mineral B

Ionic radius/nm

r

Mineral A Melt

A compatible element is more readily
accommodated in one (or more)
minerals than in the melt phase.

K i A > 1

(b)