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

ORIGIN 19
erals, which occur in the following proportions
(Eng. Mining Jour., 1952):
Percent
Titanium minerals -__-_ ______-__- 45
Staurolite .________________________________ ___ __ _... 20
Zircon __ _____________________________________._______ _ ____ ________ 15
Tourmaline ..... ... _ ._.._ 5
Sillimanite - __ -____-_-_ ___________ _ 5
Kyanite _ ___-__-______-^___________-_.___ _________________________ 4
Miscellaneous _________________________._____.________...._.________ 6
The titanium minerals, staurolite, and zircon are
separated as commercial products, but kyanite and
sillimanite have not been recovered. Recent experimental
work by the Bureau of Mines (Browning
and others, 1956) on the zircon mill tailings from
Trail Ridge and on similar material from the plant
of the Titanium Alloys Manufacturing Co. at Jacksonville
has yielded concentrates containing up to
about 96 percent combined kyanite and sillimanite
and a little corundum and zircon.
ORIGIN
GENERAL CONSIDERATIONS
The problems of origin of the Al2Si05 minerals
involve the conditions of temperature and pressure
which prevailed during metamorphism and the
chemical processes which brought about the present
chemical composition of the rock. These physical
conditions and chemical processes are only poorly
understood. Why one of the Al2Si0 5 minerals is
found instead of another in certain rocks, or why
two of them, or all three, occur together in some
rocks, is in many cases not clear. The present state
of knowledge about the conditions of formation of
these minerals is briefly reviewed below.
Turning first to the experimental work on the
synthesis of the Al2Si05 minerals, we find that
much remains to be learned about the temperature
and pressure conditions and the fields of
stability of these minerals. In early attempts at
their synthesis (Vernadsky, 1890; Shepherd and
others, 1909), a phase was formed that was first
thought to be sillimanite because of its optical properties
and slender prismatic crystal form. The
same compound was found in porcelain; it was also
formed by heating kyanite, andalusite, topaz, and
dumortierite to high temperatures (Vernadsky,
1890). However, Bowen and Greig (1924) subsequently
demonstrated that this so-called sillimanite
has the chemical composition, 3 A1203 2 Si02 ; it
was given the name "mullite" because of its occurrence
on the Island of Mull in vitrified inclusions of
aluminous sedimentary rocks in sills of mafic rock
(Bowen and others, 1924). Navais and Davey
(1925) showed that mullite and sillimanite could
be distinguished by means of their X-ray diffraction
patterns.
Experimental studies in recent years by several
groups of workers have provided a limited amount
of information about the phase equilibrium relations
of the Al2Si05 minerals. Roy and Osborn (1951,
1954) were unable to synthesize any of the Al2Si0 5
minerals in their investigation of the system
Al203-Si02-H20. They found that mullite seemed
to be the only stable alumina-silica compound at
temperatures above 575°C; pyrophyllite seemed to
crystallize between 420° and 575°C; and various
clay minerals crystallized in different temperature
ranges below 420 °C. Their colleague, Delia Roy
(1954) describes the formation of tiny crystals of
andalusite in a study of the system MgO-Al203-
Si02-H20 (see also Roy and Roy, 1955). The data
suggest that andalusite is a stable phase between
450° and 650 °C at water pressures between 10,000
and 30,000 psi; the cations Mg++ and Ca++ may
aid in the formation of andalusite.
Fyfe, Turner, and Verhoogen (1958, p. 165),
state that Goes informed them in 1954 that he had
synthesized all 3 Al2Si0 5 polymorphs at 600° to
900°C above 20,000 atmospheres; in some runs all
3 polymorphs were formed together. Kennedy (1955)
reports that sillimanite, kyanite, and pyrophyllite
are in equilibrium at 700°C and 14,000 bars; he
found that pyrophyllite breaks down to form a solid
solution series between sillimanite and mullite at
550° to 600 °C and 400 to 3,000 bars. Clark, Robertson,
and Birch (1957) succeeded in establishing
the equilibrium curve between kyanite and sillimanite
from temperature of 1,000 °C and pressure of
18,200 bars to 1,300° C and 21,000 bars; the kyanite
field is on the high pressure side of the equilibrium
curve. They found that quartz and corundum
were generally formed below 1,000 °C instead of
kyanite or sillimanite.
The geologic characteristics of kyanite, sillimanite,
and andalusite deposits give some general information
about the conditions of formation of
these minerals. Kyanite and sillimanite commonly
occur in well-foliated schists and gneisses rocks
that have been regionally metamorphosed under the
influence of heat and strong deformation. Andalusite
is more abundant in less highly deformed rocks
where recrystallization has taken place at high
temperatures.
In some areas of argillaceous rocks that have
undergone regional metamorphism, sillimanite occurs
in zones nearest igneous intrusions and kyanite