Geology Reference
In-Depth Information
texture tells us that these minerals crystallized from a
melt, which itself had 'uniform physical and chemical
properties' distinct from each of the crystalline miner-
als present. So in considering the phase relations that
dictated the present character of the rock we must
count the melt as a phase too, although it is now no
longer present as a constituent. If the basalt is
vesicular
(that is, it contains empty bubbles or 'vesicles'), we
have evidence that a sixth phase, water vapour, was
also present as the rock crystallized.
(Note that water can be present as a dissolved spe-
cies within a melt or a hydrous crystalline mineral.
Being accommodated within the volume occupied by
that phase, it does not then count as a phase in its own
right. Only when separate bubbles of water vapour
appear can one accord water the status of a separate
phase. When this happens, as with the vesiculating
lava, it is a sign that all other phases present contain as
much water as they can accommodate, and the system
is
saturated
with water.)
One must therefore be careful not to overlook addit-
ional phases which - though no longer present in a
particular rock - may have influenced its formation or
development. Here are some examples where this
could happen:
essential distinction between them is crystallographic
structure, not their chemical composition (which sev-
eral minerals may have in common). Any molten phase
present - regardless of composition - is referred to as
'melt'. By convention one refers to any gaseous phase
involved as '
vapour
'.
Component
The basic chemical constituents of a system, of which
the various phases are composed, are called its
compo-
nents
. The concept of a component is defined in a pre-
cise but rather roundabout way: 'the components of a
system comprise the
minimum number
of chemical
(atomic and molecular) species required to specify
completely the compositions of all the phases
present'.
Consider a crystal of olivine, which at its simplest
consists of the elements magnesium (chemical symbol
Mg), iron (Fe), silicon (Si) and oxygen (O). One way to
define the components of the olivine would be to
regard each chemical element as a separate component
because the composition of any olivine can be stated in
terms of the concentrations
1
of four elements:
Mg Fe Si O
(a) Some igneous rocks show textural evidence for the
existence of two distinct (presumably immiscible)
silicate liquids that once existed together in mutual
equilibrium.
(b) A metamorphic rock may have developed in the
presence of a vapour phase, permeating the grain
boundaries between crystals, of which no visible
trace now remains.
(c) Mineral veins are deposited from a fluid phase
preserved only in occasional microscopic inclu-
sions within crystals (Box 4.6).
(d) When a magma is produced by melting deep
inside the Earth, it may be in pressure-dependent
equilibrium with minerals quite different from
those that crystallize from the same melt at the
surface.
However, defining the components in this way
fails to recognize an important property of all sili-
cate minerals, including olivine: that the oxygen
content is not an independent quantity, but is tied by
valency (Chapter 6) to the amounts of Mg, Fe and Si
present, being just sufficient to generate the oxides
of each of these elements (this is explained in
Box 8.4). So, in describing the composition of an
olivine the same information can be conveyed more
economically in terms of the concentrations of only
three components:
MgOFeO SiO
2
By using a property specific to olivines, however, a
still more economical statement of olivine composition
can be devised. The crystal chemistry of olivine (see
Chapter 8) requires an olivine composition to conform
to a general formula that we can represent by X
2
SiO
4
. X
represents a type of atomic site in the olivine crystal
Thus the minerals we see in a rock today may repre-
sent only a part (or even no part at all) of the original
phase equilibria to which the rock owes its present
constitution.
It is usual to refer to solid phases by the appropriate
mineral name - quartz, kyanite, olivine, and so on. The
The ways in which concentration can be expressed are
summarized at the beginning of Chapter 4.
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