Geology Reference
In-Depth Information
2
EQUILIBRIUM IN GEOLOGICAL
SYSTEMS
The significance of mineral stability
a rock only if the vapour pressure present during its
formation falls within the appropriate range.
The sensitivity of such minerals to the physical cir-
cumstances of their formation offers the petrologist
tremendous opportunities because, when found in an
igneous or metamorphic rock now exposed at the sur-
face, they provide a means of establishing quantita-
tively the characteristics of the physical environment
in which that rock originally crystallized. The study of
mineral stability therefore offers the key to a veritable
library of information, sitting in the rocks waiting to be
utilized, about conditions and processes deep within
the Earth's crust and upper mantle.
The usual way to establish the physical limits within
which a mineral is stable - and beyond which it is
unstable - is to cook it up in a laboratory experiment
(Box 2.1). Technology today is capable of reproducing in
the laboratory the physical conditions (temperature
T
, pressure
P
, water vapour pressure
P
HO
2
Igneous and metamorphic rocks form in places that are
not, on the whole, directly accessible to the investigat-
ing geologist. To discover how such rocks are produced
within the Earth, one must resort to indirect lines of
enquiry. The most important clues consist of the min-
erals that the rocks themselves contain. A given min-
eral crystallizes as a stable phase only within restricted
ranges of pressure and temperature, as we saw in the
case of aragonite which occurs
stably
only at high pres-
sures (Figure 1.3a). Subjecting the mineral to condi-
tions that fall outside its stability range will cause
another mineral, which is stable under those condi-
tions (like calcite at low pressure), to begin crystalliz-
ing in its place. The stability of other types of minerals
may depend in a similar way on the pressure of water
vapour or some other gaseous component present
during crystallization, and such a mineral will occur in
, and so on)
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