Geology Reference
In-Depth Information
T /°C
P H 2 O = 2 × 10 8 Pa
Melt + Na-rich
feldspar ss
feldspar ss
f h
P H 2 O /10 8 Pa
f 1
f 2
% NaAlSi 3 O 8
K-rich feldspar ss +
Na-rich feldspar ss
Percentage of NaAlSi 3 O 8
by mass
KAlSi 3 O 8
NaAlSi 3 O 8
Figure 2.6 Melting and subsolidus phase relations in the alkali feldspars (the system KAlSi 3 O 8 -NaAlSi 3 O 8 ). The subscript 'ss'
denotes solid solution. (a) Perspective sketch of PTX
−− space, showing the isobaric section at 2 × 10 8 Pa illustrated in (b).
(b) Alkali feldspar phase relations at P HO
=× Pa. Horizontal ruling represents two-phase fields.
210 8
The diagram shows the liquidus and solidus of the
alkali feldspar series, which differ from those in
Figure 2.5 only in that they fall to a minimum melting
point in the middle of the series, rather than at one
end. As a result we get two leaf-shaped fields instead
of one. But interest is mainly in what happens in the
subsolidus region . The 'homogeneous feldspar ss ' field
immediately below the solidus means that here the
end-members are completely miscible in the solid
state: they form a complete solid solution in which
any composition can exist as a single homogeneous
phase. But at lower temperatures things get more
Beneath a boundary called the solvus a 'miscibility
gap' appears. At these temperatures (for example
600 °C) the albite crystal structure is less tolerant of the
KAlSi 3 O 8 component (partly because of the large size
of the potassium atom), and at a KAlSi 3 O 8 content of
about 20% ( f 2 = 80% NaAlSi 3 O 8 ) becomes saturated
with it. Any KAlSi 3 O 8 present beyond this limit is
forced to exist as a separate KAlSi 3 O 8 -rich feldspar
phase, whose composition can be found by extending
a tie-line to the left-hand limb of the solvus, cutting it
at f 1 (about 65% KAlSi 3 O 8 or 35% NaAlSi 3 O 8 ). This
potassium feldspar is itself saturated with NaAlSi 3 O 8 .
A homogeneous alkali-feldspar solid solution such
as f h ceases to be stable as it cools through the solvus. At
point f , for example, it is well inside the two-phase
region. Such a point represents, at equilibrium, a mix-
ture of two phases. The initially homogeneous feldspar
therefore breaks down, or exsolves , into two separate
phases f 1 and f 2 . But solid-state diffusion is too slow to
allow a cooling feldspar crystal to sort itself out into
two separate crystals. The usual result of exsolution is
a series of thin, lamellar domains of one phase enclosed
within a host crystal of the other. The lever rule (Box 2.3)
tells us that in the present example f 1 will be more
abundant than f 2 , and the cooling of crystal f h will there-
fore produce a host crystal of composition f l containing
exsolution lamellae of phase f 2 . Such structures are char-
acteristic of alkali feldspars, where they are known as
perthites . Figure 2.7 shows a crystal of perthite viewed
in a polarizing microscope configured to highlight this
texture. The dark streaks are albite lamellae; the lighter
host is orthoclase (divided into upper and lower por-
tions that differ in shade owing to the twinning of the
crystal). Exsolution textures analogous to perthite
(although not given this name) are developed in some
pyroxenes (Plate 3), owing to a similar miscibility gap
between diopside (CaMgSi 2 O 6 ) and enstatite (Mg 2 Si 2 O 6 ).
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