Geology Reference
In-Depth Information
Box 2.1 Phase equilibrium experiments with minerals
The process of mapping out a phase diagram like that in
Figure 2.1 is illustrated in Figure 2.1.1a. Each of the filled
and open circles represents an individual experiment in
which a sample of the relevant composition is heated in a
pressure-vessel at the pressure and temperature indicated
by the coordinates, for a sufficient time for the phases to
react and equilibrate with each other. At the end of each
experiment - which may last hours, days or even months,
depending upon the time needed to reach equilibrium - the
sample is 'quenched', meaning that it is cooled as quickly
as possible to room temperature in order to preserve the
phase assemblage formed under the conditions of the
experiment (which on slower cooling might recrystallize to
other phases - see Chapter  3). The sample is removed
from its capsule, and the phase assemblage is identified
under the microscope or by other methods. The symbol for
each experiment is ornamented on the diagram in such a
way that it indicates the nature of the phase assemblage
observed, so that the results of a series of experiments
allow the position of the phase boundary to be determined.
Conditions can be chosen for later experiments which
allow accurate bracketing of its position in P-T space.
Experiments in the laboratory must necessarily be
concluded in much shorter times than nature can take to
do the same job. Even at high temperatures, silicate
reactions are notoriously sluggish, and the assemblage
observed at the end of an experimental run might reflect
an incomplete reaction or a metastable intermediate
state rather than a true equilibrium assemblage. The pro-
portions that this problem can sometimes assume are
illustrated by the disagreement among the published
determinations of the kyanite-sillimanite-andalusite triple
point shown in Figure  2.1.1b. The present consensus
places the triple point at about 4 × 10 8 Pa and 500 °C
(Figure 2.1).
One precaution that the experimenter can take is to
ensure that the position of every phase boundary is estab-
lished by approaching it from both sides, a procedure
known as 'reversing the reaction'. In locating the kyanite-
sillimanite phase boundary, for example, it is insufficient
just to measure the temperature at which kyanite changes
into sillimanite; the careful experimenter will also meas-
ure the temperature at which sillimanite, on cooling,
inverts to kyanite.
Calcite + quartz
Wollastonite + CO 2
See reaction 2.5
Figure 2.1.1 (a) A P-T phase diagram showing the reaction curve for reaction 2.5. Each dot represents the P-T values
for an individual experiment. All experiments were conducted in a CO 2 atmosphere; the pressure of CO 2 gas present
(symbolized as P CO 2 ) is equal to the applied pressure. Solid dots identify runs that produced calcite and quartz;
open dots represent runs in which wollastonite was formed. The curve is drawn to run between filled and open dots.
(b) Differences between published P-T values of the kyanite-sillimanite-andalusite triple point shown in Figure 2.1.
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