EQUILIBRIUM IN GEOLOGICAL SYSTEMS - Chemical Fundamentals of Geology and Environmental Geoscience - page 34

Box 2.6 Reaction points and incongruent melting

Every geology student knows that olivine and quartz are
incompatible, and do not coexist stably in nature. (In fact,
this is true only of magnesium-rich forsteritic olivines.
Fayalite - Fe 2 SiO 4 - is quite a common mineral in granites
and quartz syenites.) how is this incompatibility expressed
in a phase diagram?
The relevant part of the system Mg 2 SiO 4 -SiO 2 (omitting
complications at the SiO 2 -rich end) is shown for atmos-
pheric pressure in Figure 2.6.1. In many respects it is sim-
ilar to Figure 2.4. The difference is that, between Mg 2 SiO 4
and SiO 2 along the composition axis, lies the composition
of the pyroxene enstatite, Mg 2 Si 2 O 6 . Consider the crystal-
lization of melt composition m 1 . On reaching the liquidus it
will begin to crystallize olivine, whereupon further cooling
and crystallization will lead the melt composition down the
liquidus curve. On reaching R, the melt composition has
become too SiO 2 -rich (more so than enstatite) to coexist
stably with olivine, which therefore reacts with the SiO 2 in
the melt to form crystals of enstatite:

point like E. It is called a reaction point . Temperature and
melt composition remain constant as the reaction pro-
ceeds (from left to right in reaction 2.6.1), until one or
other phase is exhausted. In this case (beginning with m 1 )
the melt is used up first, and the final result is a mixture
of olivine and enstatite: the melt never makes it to the
eutectic. If, on the other hand, the initial melt had the
composition m 2 , more siliceous than enstatite, the reac-
tion at R would transform all of the olivine into enstatite,
with some melt left over. The disappearance of olivine
releases the system from invariant equilibrium R, and the
melt can proceed down the remaining liquidus curve, crys-
tallizing enstatite directly until the eutectic is reached. The
final result is a mixture of enstatite and silica (the high-
temperature polymorph cristobalite). The proportions in
the final mixture can be worked out by applying the Lever
Rule to m 2 .
During melting, this reaction relationship manifests
itself as a phenomenon called incongruent melting.
Pure enstatite, when heated, does not melt like olivine
or anorthite but decomposes at 1557 °C to form olivine
(less SiO 2 -rich) and melt (more SiO 2 -rich than itself),
i.e. the reaction 2.6.1 run in reverse. The system is
held in invariant three-phase equilibrium until the
enstatite has been exhausted, then continues melting
by progressive incorporation of olivine into the melt
(cf. Figure 2.4).

Mg SiO iO + → Mg SiO

2 4 2 2 6

(2.6.1)

olivine elt pyroxene

(This symbolism does not mean that the melt consists of
SiO 2 alone. Other components are present, but this reac-
tion involves only the SiO 2 component.)
At R, the three phases are at equilibrium. Using the
Condensed Phase Rule, it is clear that R is an invariant

1650

Melt

m 1

1600

Melt + Fo

Melt + En

m 2

R

Melt + Silica

1557

1550

E

1543

En + Fo

Silica + En

70 100

50 60 80 90

Figure 2.6.1 The Mg-rich part of the system
Mg 2 SiO 4 -SiO 2 showing the reaction point R
between Mg 2 SiO 4 (forsterite) and SiO 2 -rich
melts (reaction 2.6.1).

Mass percentage of Mg 2 SiO 4

SiO 2 Mg 2 Si 2 O 6 (En) Mg 2 SiO 4 (Fo)