Geology Reference
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encountered anywhere in the crust and in most of the
upper mantle. Experiments can be conducted in which
minerals are synthesized at a series of accurately known
temperatures and pressures, from starting materials of
known composition. In each case, the phases (minerals)
formed are considered to crystallize in chemical equilib-
rium with each other, and/or with a molten silicate
liquid, and for this reason the experiments are called
phase-equilibrium experiments . Experimental information
like this is vital to the petrological interpretation of
naturally occurring rocks. The results are used to map
out, in phase diagrams similar to Figure 1.3a, the areas
within which particular mineral assemblages are stable.
In crossing a boundary from one stability field into a
neighbouring one as physical conditions change, one
mineral assemblage recrystallizes into another, a chem-
ical reaction between coexisting minerals which trans-
forms an unstable - or metastable - assemblage into a
new, stable one. These boundaries, like the diagonal
line in Figure 1.3a, are called phase boundaries or, more
generally, reaction boundaries .
Phase diagrams form a key part of the literature of
petrology, a powerful means of portraying and inter-
preting the phase-equilibrium data relevant to igneous
and metamorphic petrogenesis. Reading and inter-
preting such diagrams, in the light of underlying
thermodynamic principles, is one of the basic skills
essential to every geologist.
Just as atmospheric pressure represents the weight
of the column of air above us under the influence the
Earth's gravitational field, so the pressure experienced
by a rock buried at some depth within the Earth reflects
the total weight of the column of rock + ocean + atmos-
phere resting upon it. This lithostatic pressure P there-
fore increases with depth d in a simple manner
conveniently approximated as:
This is a handy word for describing any part of the
world to which we wish to confine attention.
Depending on the context, system could mean the
whole of the Earth's crust, or the oceans, or a cooling
magma chamber, or an individual rock, or a sample
undergoing a phase-equilibrium experiment. In most
cases the term will refer to a collection of geological
phases (see below) interacting with each other.
An open system is one that is free to exchange both
material and energy with its surroundings. The sea is
an open system; it may be useful to consider it as an
isolated entity for the purposes of discussion, but one
must recognize that it receives both sunlight and river
water from outside, and loses heat, water vapour and
sediment to the atmosphere and crust.
A closed system is one that is sealed with respect to
the transfer of matter, but that can still exchange energy
with the surroundings. A sealed magma chamber
would be a good example, its only interaction with its
environment (ideally) being the gradual loss of heat.
An isolated system is one that exchanges neither mass
nor energy with its surroundings, a notion of little
relevance to the real world.
'System' may alternatively be used to denote a domain
of chemical (rather than physical) space. Petrologists use
the word to distinguish a particular region of ' composi-
tional space ' to which attention is to be confined. Thus
one speaks of 'the system MgO-SiO 2 ', referring to the
series of compositions that can be generated by mixing
these two chemical components in all possible pro-
portions. The system so designated includes various
minerals whose compositions lie within this range (the
silica minerals, and olivine and pyroxene).
The meaning of phase in physical chemistry and petrol-
ogy is easy to grasp although cumbersome to put into
words. In formal terms, a phase can be defined as 'a
part or parts of a system occupying a specific volume
and having uniform physical and chemical characteristics
which distinguish it from all other parts of the system'.
Each individual mineral in a rock thus constitutes a
separate phase, but that is not the end of the story. A
lump of basalt collected from a solidified lava flow
might be found to consist of four minerals, say plagio-
clase, augite, olivine and magnetite. But the igneous
where P is expressed in units of 10 8 Pa and d is in km.
The high pressure applied in some phase-equilibrium
experiments is simply the means by which we simu-
late in the laboratory the effect of depth in the Earth.
Systems, phases and components
To avoid confusion, one must be clear about the mean-
ing of several terms which are used in a specific sense
in the context of phase equilibrium.
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