Geoscience Reference
In-Depth Information
1800
1800
T
A
A
1700
1700
1600
1600
1500
1500
Liquid
1400
1400
1300
1300
Liquid
+
quartz
Liquid
+
albite
1200
1200
E
1100
1100
Quartz
+
albite
1000
1000
x
A
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Mole fraction of albite
Figure C.2
Simplified representation of the quartz-albite binary system. The melting temperatures of quartz
and albite are 1700 K and 1373 K, and their latent heats of fusion are 9400 and 63 000 J mol
−
1
,
respectively. The liquidus curves are calculated from
(C.42)
and
(
C.43)
.
The metastable part of the
curves is shown as thin dashed lines. Their point of intersection is the eutectic point E, where a
solid and a liquid of the same composition co-exist. Evolution by cooling of a liquid of composition
x
A
and initially at temperature
T
A
, and that of a solid at equilibrium are shown by the thick solid
line (solid) and the thick dashed line (liquid). The path for melting would be the exact reverse.
which is no more favorable. The system's composition is therefore locked at the eutectic
point (constant temperature and composition). When a solid system of the same compo-
sition melts, the pathway is reversed. The first liquids are of eutectic composition until
the albite disappears. At that point the liquid evolves toward quartz and total melting is
achieved when its composition returns to that of the initial solid. We have chosen a binary
system in which solids do not form a mutual solution. Other systems whose components
crystallize in similar forms (Fe-Mg olivine, Na-Ca feldspar) have solid solutions that may
be treated in similar ways at the cost of slight complications in form. This approach is
not specific to solid-liquid equilibria. It can be extended to phase equilibria of all sorts
(solid-gas, solid-solid). Reaction phase equilibria are processed in the same way, with the
assemblages being referred to as peritectic.
