Kyanite, Sillimanite, and Andalusite Deposits of the Southeastern ...

ORIGIN 21
of temperature and pressure during complex
thermal metamorphism superimposed on a regional
metamorphism.
Using the general geologic relationships of deposits
of the Al2SiO5 minerals as a guide, Miyashiro
(1949) has attempted to express the physical conditions
of formation of these minerals by means
of a pressure-temperature diagram (fig. 24) that
TEMPERATURE
EXPLANATION
gf
Green schist facies
eaf
Epidote amphibolite facies
af
Amphibolite facies
phf
Pyroxene hornfels facies
sf
Sanidinite facies
FIGURE 24. Diagram showing Miyashiro's concept of the stability fields of
kyanite, sillimanite, andalusite, and mullite. After Miyashiro (1949).
shows the stability fields of each of the minerals.
If his diagram approximates the actual state of conditions,
then the association of 2, or all 3, of the
minerals together need not necessarily be the result
of fluctuating conditions of temperature and pressure
during metamorphism. It could be the result
of changes in only one factor, such as a change
in temperature under nearly constant pressure
or a change in pressure with little change in
temperature. Miyashiro evidently regards pressure
as being very important in the formation of these
minerals, because he suggests that "these minerals
are useful as indicators of pressures of their formation."
The opposite view of the importance of
pressure in metamorphic reactions has been expressed
by Yoder (1952, p. 617-618) in his discussion
of studies in the MgO-Al2O3-SiO2-H20
system. He believes that temperature and composition
are the important variables, and that pressure
is of minor significance.
Thompson (1955, p. 97) modifies Miyashiro's
concept of the stability relationships of the Al2SiO5
minerals by adding fields for hydrous aluminum
silicates; he believes that water may remain in the
system to higher temperatures in thermal metamorphism
than in regional metamorphism, which
may delay or prevent the formation of kyanite from
the hydrous aluminum silicates in thermal metamorphism
(see next paragraph). Schuiling (1957,
1958) also proposes a phase diagram that is very
similar to those of Miyashiro and Thompson but
goes a step farther by inferring values for temperature
and pressure. Clark, Robertson, and Birch
(1957, 1958) suggest a phase diagram in which
the triple point of the Al2SiO 5 polymorphs lies at
a higher pressure and lower temperature than in
Schuiling's diagram. The phase diagram of Miyashiro
(fig. 24) and its several variations just discussed
all envision sillimanite as the high-temperature
polymorph. Fyfe and others (1958, p. 164-165)
offer two other diagrams of the hypothetical relations
of the AloSiO5 minerals in which sillimanite
is regarded as the low-temperature form in one
diagram and as the intermediate-temperature form
in the other; these proposed diagrams do not seem
to be so well based on experimental data or field
relations as Miyashiro's diagram.
Water is generally present in rocks that are undergoing
metamorphism, and is essential in the
metamorphic processes. Aqueous solutions or gases
are diffused through the rocks, and provide a medium
that facilitates mineral recrystallization in
response to temperature changes. Yoder (1952, p.
614-617) shows that in the MgO-Al2O3-SiO2-H2O
system under closed conditions, changes of only a
few percent in the water content may cause important
changes in the mineralogy. The pressure of
water (and also of C0 2 ) is probably a significant
factor in metamorphism, along with load pressure
and temperature, according to Fyfe and others
(1958, p. 15-17). Thompson (1955, p. 96-101)
points out that the chances for water to escape are
better under conditions of regional metamorphism
than of thermal metamorphism, because the stronger
deformation that accompanies regional metamorphism
should tend to keep channels open for move-