Geoscience Reference
In-Depth Information
2.3 Films and interfaces
Although they are essentially made of minerals, most rocks also contain some interstitial
material, which may evade precise analysis when the scale of the constituting phases is
too small, typically below 10 nanometers (1 nm
10
−
9
m). This is the case for cement in
sedimentary rocks, but also for common interstitial phases of igneous rocks and metamor-
phic rocks, such as apatite, oxides, carbonates, clay minerals, which have been deposited
by the percolation of aqueous or carbonated fluids. Peridotite xenoliths in basaltic rocks
often look shiny because of the presence of an interstitial film of magmatic glass. It is
also now well understood that a cryptic mechanism of diffusive segregation often concen-
trates elements incompatible with the structure of olivine from mantle rocks, Ti, Ca, Al, as
ill-defined mineral phases no larger than a few nm.
Two major questions arise with interfaces and films. First, the energy of small minerals
should be understood through the sum of two terms: volume energy, which is the only form
we have been considering until now, and surface energy, which is the extra energy needed to
produce the dangling bonds. It is therefore inconvenient to treat phases that take up hardly
more than a few lattice units as we would treat large crystals. Second, we may wonder
whether minute phases and films account for a substantial proportion of the elementary
budget of a rock sample. Let us take an ideal rock formed of spherical minerals from a
single species, e.g. olivine, and with homogeneous radii
a
. We assume that these minerals
are homogeneously covered with a film of thickness
=
a
. Let us consider an element with a
whole-rock concentration
C
0
, a concentration
C
R
in the mineral, and a partition coefficient
D
f
between the interstitial film and the mineral. In order to calculate the contribution of the
film to the total inventory of the rock for this element, we first note that the mass fraction
F
f
of the film is
δ
a
2
4
π
δ
a
3
δ
a
/
a
F
f
=
a
=
(2.18)
4
1
+
3
δ
a
/
a
3
π
a
3
+
4
π
a
2
δ
and then write the mass balance as:
C
0
=
(1
−
F
f
)
C
R
+
F
f
D
f
C
R
(2.19)
Since
F
f
1, it is clear that the impact of the film on the rock budget becomes noticeable
only when
F
f
D
f
≈
1. For a film of 100 nm wrapped around olivine crystals with a radius
of 1 mm, the effect is only visible for elements with
D
f
10
4
. This is potentially harmful
for large-ion lithophile elements (LILE) such as U, Th, and Ba: the external part of minerals
must be carefully taken away before analysis. It is standard practice for this type of analysis
to remove mineral grains with exposed interface under the optical microscope and then
to submit the residue to leaching (
>
partial dissolution in strong acids) before complete
dissolution. The ground truth, however, is that situations in which most of the element
inventory resides in the interstitial phase (
D
f
F
f
=
1), with sample chemistry being driven
by the interstitial phase, are exceedingly rare.
